Delete gopath-test taskrun example / YAML test

This example doesn't demonstrate much, and it's the longest-running YAML
test at about 3-5 minutes. It was copied over from knative/build days
and hasn't seen many updates since.

The example clones github.com/tektoncd/pipeline into a targetPath within
$GOPATH and runs `go test ./...` on it, which is what takes so long.

If we decide this example is worth keeping around, we should probably at
least change the repo to something smaller and quicker to test, or
change `go test` to just `go build` to save time.

Author: imjasonh