Use exported gtest include directories

Manually inserting gtest include directories causes build failures in
manifests that don't have external/gtest.  external/gtest already
exports its include directories so remove the manual include directory.

Change-Id: I6baac4bd3bea7dba3aa134d18b66f02a6dc9a54a

cc/cc.go

@@ -1997,10 +1997,6 @@ func (test *testLinker) flags(ctx ModuleContext, flags Flags) Flags {
 		flags.CFlags = append(flags.CFlags, "-DGTEST_OS_LINUX_ANDROID")
 	}
 
-	// TODO(danalbert): Make gtest export its dependencies.
-	flags.CFlags = append(flags.CFlags,
-		"-I"+common.PathForSource(ctx, "external/gtest/include").String())
-
 	return flags
 }