Re-remove snapshot[1234] from mediawiki-installation dsh group

The snapshot[1234] servers in pmtpa were removed from the dsh group in
I30ec535 but reintroduced unintentionally in Ie3548a3.

Change-Id: Ib12fcd68cb1775b2db23effb55a176e070cf0fe5
Merge "Re-remove snapshot[1234] from mediawiki-installation dsh group" into production