applicationserver: rm code for labs instance in pmtpa

The pmtpa instances had to mount /dev/vdb to /srv instead of /mnt.  The
beta cluster project has been migrated to eqiad which no more use
/dev/vdb.

This patch clean up the pmtpa/labs case in role::applicationserver.

Change-Id: Iecac4746f18bba8efa16d08a3e31786ec1ba2af6
Merge "applicationserver: rm code for labs instance in pmtpa" into production