cmd/go: remove TestScript/get_unicode and an internal get test case

That test tested that import paths with non-ASCII unicode paths
were allowed by the Go command. Remove this test case because
golang.org/cl/251878 removes that support.

Also rewrite a test case in TestRepoRootForImportPath in the test
for cmd/go/internal/get to reflect that unicode directory names are now
disallowed.

Updates #29101

Change-Id: I669e220facd04fc82ccd05dd08e8f1ff4d48b1fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/252297


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>