Remove wikimedia-task-appserver from app servers

The wikimedia-task-appserver package does the following things:

* Pulls in packages required for running MediaWiki
* Configure the apache user
* Configure the pybal user

All of these things are now expressed in Puppet, so we can finally
get rid of it.

I ran 'apt-mark manual' on the mediawiki-installation dsh group for those
packages that are explicitly declared as present in mediawiki::packages.
This is necessary to ensure that they are not marked as candidates for
autoremoval once wikimedia-task-appserver is removed.

Change-Id: Ie5a860eb94c67896cf2b6764c173908c4b2e65a4
Merge "Remove wikimedia-task-appserver from app servers" into production