cmd/dist: drop unneeded clang flags

Our source no longer needs these flags set to build cleanly using clang.

Tested with

* Ubuntu clang version 3.0-6ubuntu3 (tags/RELEASE_30/final) (based on LLVM 3.0) on i386
* clang version 3.2 (tags/RELEASE_32/final) on amd64 cross compiling all platforms

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7058053

src/cmd/dist/build.c

@@ -601,10 +601,6 @@ install(char *dir)
 		splitfields(&gccargs, bstr(&b));
 		for(i=0; i<nelem(proto_gccargs); i++)
 			vadd(&gccargs, proto_gccargs[i]);
-		if(xstrstr(bstr(&b), "clang") != nil) {
-			vadd(&gccargs, "-Wno-dangling-else");
-			vadd(&gccargs, "-Wno-unused-value");
-		}
 	}
 
 	islib = hasprefix(dir, "lib") || streq(dir, "cmd/cc") || streq(dir, "cmd/gc");