Syntax:
DROP FUNCTION [IF EXISTS] function_name

This statement drops the loadable function 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 functions
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-loadable.html

