fix(doc): remove duplicate agg func doc

Signed-off-by: Jiyong Huang <huangjy@emqx.io>

docs/en_US/sqls/built-in_functions.md

@@ -22,12 +22,6 @@ Aggregate functions perform a calculation on a set of values and return a single
 | vars            | vars(col)                        | Returns the sample variance (square of the sample standard deviation) of expression in the group, usually a window. The argument is the column as the key to vars.                                                                                                                                                                                                                                                                |
 | percentile      | percentile(col, percentile)      | Returns the percentile value based on a continuous distribution of expression in the group, usually a window. The first argument is the column as the key to percentile.  The second argument is the percentile of the value that you want to find. The percentile must be a constant between 0.0 and 1.0.                                                                                                                        |
 | percentile_disc | percentile_disc(col, percentile) | Returns the percentile value based on a discrete distribution of expression in the group, usually a window. The first argument is the column as the key to percentile_disc.  The second argument is the percentile of the value that you want to find. The percentile must be a constant between 0.0 and 1.0.                                                                                                                     |
-| stddev          | stddev(col)                      | Returns the population standard deviation of expression in the group, usually a window. The argument is the column as the key to stddev.                                                                                                                                                                                                                                                                                          |
-| stddevs         | stddevs(col)                     | Returns the sample standard deviation of expression in the group, usually a window. The argument is the column as the key to stddevs.                                                                                                                                                                                                                                                                                             |
-| var             | var(col)                         | Returns the population variance (square of the population standard deviation) of expression in the group, usually a window. The argument is the column as the key to var.                                                                                                                                                                                                                                                         |
-| vars            | vars(col)                        | Returns the sample variance (square of the sample standard deviation) of expression in the group, usually a window. The argument is the column as the key to vars.                                                                                                                                                                                                                                                                |
-| percentile      | percentile(col, percentile)      | Returns the percentile value based on a continuous distribution of expression in the group, usually a window. The first argument is the column as the key to percentile.  The second argument is the percentile of the value that you want to find. The percentile must be a constant between 0.0 and 1.0.                                                                                                                        |
-| percentile_disc | percentile_disc(col, percentile) | Returns the percentile value based on a discrete distribution of expression in the group, usually a window. The first argument is the column as the key to percentile_disc.  The second argument is the percentile of the value that you want to find. The percentile must be a constant between 0.0 and 1.0.                                                                                                                     |
 
 ### Collect() Examples