Remove all pmtpa hosts from dsh scap-proxies group

Follow up to Ie3548a3 and Ic56177a. After then first scap that included
these changes we started seeing errors from srv270, mw40 and
semi-unexplainably mw31 caused by the removal of the db-pmtpa.php
configuration file.

This patch will stop updating the former pmtpa scap rsync proxies. When
this has been merged,
`touch /usr/local/apache/common-local/wmf-config/db-ptmpa.php` should be
performed on srv270, mw40 and mw31 to stop php fatal errors during
icinga health checks. Other ptmpa hosts should be fine as they didn't
receive the Ic56177a deletion.

Change-Id: I332c0e89432ac23e64cef90500490939a505bbd2