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>

src/cmd/go/internal/get/vcs_test.go

@@ -32,13 +32,10 @@ func TestRepoRootForImportPath(t *testing.T) {
 				Repo: "https://github.com/golang/groupcache",
 			},
 		},
-		// Unicode letters in directories (issue 18660).
+		// Unicode letters in directories are not valid.
 		{
 			"github.com/user/unicode/испытание",
-			&RepoRoot{
-				vcs:  vcsGit,
-				Repo: "https://github.com/user/unicode",
-			},
+			nil,
 		},
 		// IBM DevOps Services tests
 		{