Remove unused mysql config (#16080)

docker/etc/mysql/conf.d/mysql.cnf

@@ -1,6 +0,0 @@
-[mysqld]
-init_connect='SET collation_connection = utf8_unicode_ci'
-init_connect='SET NAMES utf8'
-character-set-server=utf8
-collation-server=utf8_unicode_ci
-skip-character-set-client-handshake