cmd/go: don't pass -compiler flag to vet

Without this running go vet -compiler=gccgo causes vet to fail.
The vet tool does need to know the compiler, but it is passed in
vetConfig.Compiler.

Change-Id: If857be4f336f6d7c425972fabcf82fae2cdc8a90
Reviewed-on: https://go-review.googlesource.com/113715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/vet/vetflag.go

@@ -90,7 +90,7 @@ func vetFlags(args []string) (passToVet, packageNames []string) {
 			}
 			switch f.Name {
 			// Flags known to the build but not to vet, so must be dropped.
-			case "x", "n", "vettool":
+			case "x", "n", "vettool", "compiler":
 				if extraWord {
 					args = append(args[:i], args[i+2:]...)
 					extraWord = false