DatabaseMysqlBase: Remove broken check for Percona Server

Percona Server does not include its name in its version suffix.
It would be possible to check for precompiled versions of the
product using the version_comment variable, though not third-
party builds from its source code (which are merely indicated
as "Source distribution").

Given that even Percona itself does not distinguish between
Percona Server and MySQL in their version of the mysql command-
line client, I have chosen simply to remove the broken check.

I have also made the check for MariaDB consistent with that
in its version of mysql (by also checking for the old suffix
"-maria-").

Follows-up 0eab1ace67e7.

Change-Id: I62c3949b775a38f28f09568ebb28af2adb9be69b
Merge "DatabaseMysqlBase: Remove broken check for Percona Server"