Remove Unnecessary mysql::server Dependency

Previously, the db::mysql class required mysql::server.  This is
unnecessary since the mysql::db define requires the mysql::config
class for db creattion.  Additionally, this prevents users from
using a different class such as galera::server to manage
the database.

Change-Id: Ief246d868bcee0f6d1eabd873ad90dac56aa1ee6