Remove documentation for old MySQL server

We're supporting 5.5 and newer, while this has been changed in 5.1.

Issue #13062

Signed-off-by: Michal Čihař <michal@cihar.com>

libraries/server_privileges.lib.php

@@ -4607,7 +4607,6 @@ function PMA_getHtmlForUserOverview($pmaThemeImage, $text_dir)
             $raw = 'Your privilege table structure seems to be older than'
                 . ' this MySQL version!<br />'
                 . 'Please run the <code>mysql_upgrade</code> command'
-                . '(<code>mysql_fix_privilege_tables</code> on older systems)'
                 . ' that should be included in your MySQL server distribution'
                 . ' to solve this problem!';
             $html_output .= Message::rawError($raw)->getDisplay();