Remove control link from PMA_List_Database class

This code is there from time when we used SHOW DATABASE, which could
fail with --skip-show-database. As we now use information schema which
is not limited by this, there is no need for this workaround.

Signed-off-by: Michal Čihař <michal@cihar.com>
Merge pull request #963 from nijel/remove-skip-show-database

Remove control link from PMA_List_Database class