MDEV-39777: Fixed flaky plugins.unix_socket test

revert incorrect fix, suppress warning at DROP USER instead

Author: vuvova

mysql-test/suite/plugins/t/unix_socket.test

@@ -16,9 +16,6 @@ eval grant select on test.* to '$USER';
 --let $replace2=$USER@%
 --replace_result $replace1 "USER@localhost" $replace2 "USER@%"
 select user(), current_user(), database();
-# Enforce the cleanup so the next execution of in DROP USER
-# doesn't see this connection active.
---ping
 EOF
 
 --echo #
@@ -32,9 +29,11 @@ EOF
 --error 1
 --exec $MYSQL_TEST -u foobar < $MYSQLTEST_VARDIR/tmp/peercred_test.txt
 
+--disable_warnings
 --let $replace=drop user '$USER'
 --replace_result $replace "drop user 'USER'"
 eval drop user '$USER';
+--enable_warnings
 
 --echo #
 --echo # and now with anonymous user