Stop passing the gold plugin path to llvm-ar.

llvm-ar takes a --plugin argument but it is ignored for compatibility,
so passing it has no effect.

Change-Id: I4fc51d226d66cf2a43462d3d4ccc12e6e5ebb226

cc/util.go

@@ -86,7 +86,6 @@ func flagsToBuilderFlags(in Flags) builderFlags {
 		systemIncludeFlags: strings.Join(in.SystemIncludeFlags, " "),
 
 		groupStaticLibs: in.GroupStaticLibs,
-		arGoldPlugin:    in.ArGoldPlugin,
 	}
 }