Remove libcxx.mk.

We've had proper build system support for this for a long time.

Change-Id: I0ff2309503d44aba4940973af374fc7becde4d9c

libcxx.mk

@@ -1,19 +0,0 @@
-#
-# Copyright (C) 2014 The Android Open Source Project
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-#
-
-# New projects shouldn't use this file. To use libc++, add the following line to
-# your makefile.
-LOCAL_CXX_STL := libc++