cmd/go: remove 'go tool benchcmp' and corresponding special case

See the discussion on the original CL for context:
        https://golang.org/cl/60100043/

LGTM=josharian
R=r, josharian
CC=golang-codereviews
https://golang.org/cl/68890043

doc/go1.3.txt

@@ -5,7 +5,7 @@ runtime: output how long goroutines are blocked (CL 50420043)
 syscall: add NewCallbackCDecl to use for windows callbacks (CL 36180044)
 testing: diagnose buggy tests that panic(nil) (CL 55780043)
 testing: add b.RunParallel function (CL 57270043)
-misc/benchcmp has been replaced by go tool benchcmp (CL 47980043)
+misc/benchcmp: removed and replaced by go.tools/cmd/benchcmp (CL 47980043)
 cmd/go, go/build: support .m files (CL 60590044)
 unicode: upgrade from Unicode 6.2.0 to 6.3.0 (CL 65400044)
 runtime/debug: add SetPanicOnFault (CL 66590044)

src/cmd/go/pkg.go

@@ -315,7 +315,6 @@ var goTools = map[string]targetDir{
 	"cmd/nm":                               toTool,
 	"cmd/pack":                             toTool,
 	"cmd/yacc":                             toTool,
-	"code.google.com/p/go.tools/cmd/benchcmp": toTool,
 	"code.google.com/p/go.tools/cmd/cover": toTool,
 	"code.google.com/p/go.tools/cmd/godoc": toBin,
 	"code.google.com/p/go.tools/cmd/vet":   toTool,

src/cmd/go/tool.go

@@ -65,7 +65,7 @@ func tool(toolName string) string {
 
 func isInGoToolsRepo(toolName string) bool {
 	switch toolName {
-	case "benchcmp", "cover", "vet":
+	case "cover", "vet":
 		return true
 	}
 	return false