[dev.cmdgo] cmd: update x/tools and remove copy of txtar

golang.org/x/tools/txtar is the main location for this package. We
don't need our own copy.

Also, update cmd/vet tests for compatibility with CL 301949.

For golang/go#47193

Change-Id: I480eb591f57a0d05b433a657653e2021e39354eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/337352


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>