Remove third-party cityhash, unused since r8992

src/utils/SkCityHash.cpp

-/*
- * Copyright 2012 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-/**
- * Pass any calls through to the CityHash library.
- * This is the only source file that accesses the CityHash code directly.
- */
-
-#include "SkCityHash.h"
-#include "SkTypes.h"
-#include "city.h"
-
-uint32_t SkCityHash::Compute32(const char *data, size_t size) {
-    return CityHash32(data, size);
-}
-
-uint64_t SkCityHash::Compute64(const char *data, size_t size) {
-    return CityHash64(data, size);
-}