cirrus: Remove Linux tests

The Linux tests under the Cirrus CI are currently brittle due to
environmental issues. They're also redundant, since Linux testing is
much better covered by the GitLab CI tests.

So this patch removes them, which removes the false positives and speeds
up the Cirrus CI runs.

.cirrus.yml

@@ -4,29 +4,10 @@
 env:
   GOPROXY: https://proxy.golang.org
 
-# Run go tests under a Linux container.
+# 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.
-linux_task:
-  name: Go tests (Linux)
-  container:
-    image: golang:latest
-  modules_cache:
-    fingerprint_script: cat go.sum
-    folder: $GOPATH/pkg/mod
-  get_script: |
-    useradd --create-home --user-group testing
-    chown -R testing:testing $GOPATH/
-    su testing -c "go mod download"
-    su testing -c "go get ./..."
-  build_script: |
-    su testing -c "go build ./..."
-  test_script: |
-    su testing -c "go test ./..."
-
-# Run go tests under a FreeBSD VM.
-# Same as linux_task, we need to use a separate non-root user.
 freebsd_task:
   name: Go tests (FreeBSD)
   freebsd_instance: