cmd/go: delete unused function isInGoToolsRepo

The function isInGoToolsRepo has no use case anymore, so we remove it
with this change.

Change-Id: I71051828eef2e317b83e13f83a14d5f0bc0ec13f
Reviewed-on: https://go-review.googlesource.com/66350Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/base/tool.go

@@ -36,18 +36,9 @@ func Tool(toolName string) string {
 	}
 	// Give a nice message if there is no tool with that name.
 	if _, err := os.Stat(toolPath); err != nil {
-		if isInGoToolsRepo(toolName) {
-			fmt.Fprintf(os.Stderr, "go tool: no such tool %q; to install:\n\tgo get golang.org/x/tools/cmd/%s\n", toolName, toolName)
-		} else {
-			fmt.Fprintf(os.Stderr, "go tool: no such tool %q\n", toolName)
-		}
+		fmt.Fprintf(os.Stderr, "go tool: no such tool %q\n", toolName)
 		SetExitStatus(2)
 		Exit()
 	}
 	return toolPath
 }
-
-// TODO: Delete.
-func isInGoToolsRepo(toolName string) bool {
-	return false
-}