cmd/vendor: eliminate vet-lite

cmd/vet, now simplified to a single function call is now authoritative,
not a copy of vet-lite.

The update-xtools.sh script now uses the imports of cmd/vet as the
roots for vendoring.

Change-Id: I4faef3fcf3db10b3a3930726e8d0720a3c8395da
Reviewed-on: https://go-review.googlesource.com/c/150297


Run-TryBot: Alan Donovan <adonovan@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>