Disable the pkg-deinstall script.

spamd's pkg-install adds spamd stuff to /etc/services, and pkg-deinstall
removed it. The problem is that pkg doesn't run DEINSTALL before INSTALL
when upgrading/reinstalling. As a result, when spamd is restarted,
/etc/services winds up lacking the spamd entries, causing spamd to fail
to start.

The ideal solution is pkg running those targets in the predicted order,
or pkg gaining a @services keyword. In the meantime, this commit just
disables the pkg-deinstall. If you are uninstalling spamd you can remove
those /etc/services entries by hand.

PR: 212335
Approved by: maintainer timeout (~ 1 yr)
MFH: 2017Q3