Syntax:
DROP FUNCTION [IF EXISTS] function_name

This statement drops the user-defined function (UDF) named
function_name. (DROP FUNCTION is also used to drop stored functions;
see [HELP DROP PROCEDURE].)

DROP FUNCTION is the complement of CREATE FUNCTION. It requires the
DELETE privilege for the mysql system database because it removes the
row from the mysql.func system table that registers the function.

During the normal startup sequence, the server loads UDFs registered in
the mysql.func table. Because DROP FUNCTION removes the mysql.func row
for the dropped function, the server does not load the function during
subsequent restarts.

URL: https://dev.mysql.com/doc/refman/5.7/en/drop-function-udf.html

