Android: Removed custom toolchain file

This commit drops the previous custom CMake toolchain file for Android
in favor of CMake's new built-in toolchain for this (CMake >3.7.2).

This makes building SFML for Android a lot simpler and more straight
forward, working almost as smooth as other platforms.

To configure your build directory, all you have to do is defining just a
few variables the first time you invoke CMake.

**Required Variables**

* `CMAKE_SYSTEM_NAME` must be `Android`, so CMake knows we actually want
  to cross-compile.
* `CMAKE_ANDROID_NDK` must point to the NDK's installation directory,
  e.g. `/usr/android/ndk` or `c:/android/ndk`.

**Recommended Variables**
* `CMAKE_ANDROID_STL_TYPE` defines the STL implementation to be used.
  You should use `c++_shared`, although others might work.

**Optional Variables**
* `CMAKE_SYSTEM_VERSION` can be set to pick a specific SDK version other
  than the latest.
* `CMAKE_ANDROID_ARCH_ABI` defines the target architecture and ABI, for
  example `armeabi` or `armeabi-v7a`.

Based on your system, you might want to enforce a specific generator to
prevent issues, e.g. using `MinGW Makefiles`.

include/SFML/OpenGL.hpp

@@ -69,8 +69,6 @@
     #include <GLES/glext.h>
     
     // We're not using OpenGL ES 2+ yet, but we can use the sRGB extension
-    // We need to import gl2platform.h, would normally be included by gl2.h
-    // which was included by gl2ext.h in older NDK versions
     #include <GLES2/gl2platform.h>
     #include <GLES2/gl2ext.h>