mediawiki: use apache module

The MediaWiki module defines and manages its own apache service because it uses
apache service status as a way of detecting a server that has just been
newly provisioned or rebooted.

The way it is currently supposed to work is this:
* the Apache service is not started on boot; it depends on Puppet to start it.
* but Service['apache'] subscribes to Exec['mw-sync'], so mw-sync is run first.
* mw-sync only runs unless '/bin/ps -C apache2', so if Apache is already
  running, it doesn't run.

The idea is that the first Puppet run syncs the server and starts Apache.

I say "supposed to work" above because the sync-common script which mw-sync
invokes is a symlink to a file in /srv/scap which is manually deployed via
Trebuchet.  So a newly-provisioned server can't bootstrap itself anyhow.

I thought of fixing this comprehensively in a single commit, but it is too
risky. I think the right approach is to simply remove it for now. Keeping code
around that is known not to work is pernicious and misleading, and certainly
much worse than not having it at all.

Change-Id: I17344357a67527fe6f55eb3c9c9bda46de2cd201