cirrus: remove FreeBSD 11.4.

Many packages needed to build Wireshark are missing from pkg in
FreeBSD 11.4. They're only available in ports. That doesn't mean
that Wireshark is not compilable on FreeBSD 11.4, but that it
requires a long build from ports that is unlikely compatible with
Cirrus' CI (and its timeouts).

.cirrus.yml

@@ -4,7 +4,6 @@ task:
     matrix:
       - image_family: freebsd-13-0-snap
       - image_family: freebsd-12-1
-      - image_family: freebsd-11-4
   env:
     ASSUME_ALWAYS_YES: YES
   setup_script: