ci: Remove Cirrus CI configuration

Unfortunately, but understandably, Cirrus CI no longer offers enough
free compute credits to run our tests reliably.

They're currently only used to run the Go tests on FreeBSD.

In the future, this might be replaced with something else; but until a
proper replacement can be found and tested, remove it to avoid
introducing noise in the CI results.

.cirrus.yml

@@ -1,18 +0,0 @@
-# Configuration for https://cirrus-ci.org/
-
-env:
-  GOPROXY: https://proxy.golang.org
-
-# Run go tests under a FreeBSD VM.
-# Note that by default it is run as root, which causes the tests to fail since
-# they expect to get permission denied on some cases, so we create a separate
-# user and use that instead.
-freebsd_task:
-  name: Go tests (FreeBSD)
-  freebsd_instance:
-      image_family: freebsd-13-0-snap
-  install_script: |
-    pkg install -y lang/go
-    pw useradd testing -m
-  test_script: |
-    su testing -c "go test ./..."