cmd/go: delete testdata/testonly dir: it isn't being used anywhere
The TestGoBuildTestOnly test seems to be using files in testonly,
but it's actually creating different files in a tempdir GOPATH
that are completely unrelated.
Part of converting all tests to script framework to improve
test parallelism.
Updates #36320
Updates #17751
Change-Id: Ie2c6d477bbb2eac7c013ee8dea9330a367b4f663
Reviewed-on: https://go-review.googlesource.com/c/go/+/213821 Reviewed-by: Jay Conrod <jayconrod@google.com>