Removes Unneeded mysql::server Dependency

Previously, the heat::db::mysql class contained a dependency on
mysql::server. This dependency is unneeded since mysql::db requires
mysql::config, which depends on mysql::server. Additionally, the
mysql::server dependency causes MYSQL HA installations such as
Galera to fail. Lastly, this aligns the class more closely with
the other [OPENSTACK_SERVICE]::db::mysql classes.

Change-Id: Ie6c4d3c8aaa531502bc5b9cd66a547f1f506385f