Remove the STLPort mention from the docs and mark gnustl as deprecated

Author: var-const

release_build_files/readme.md

@@ -47,7 +47,7 @@ Directories               | Contents
 include                   | C++ headers
 frameworks/ios/ARCH       | iOS frameworks (compiled against libc++)<br>A multi-architecture framework is<br>provided in the *universal* directory.
 libs/ios/ARCH             | iOS static libraries (compiled against<br>libc++)<br>Multi-architecture libraries are<br/>provided in the *universal* directory.
-libs/android/ARCH/STL     | Android (GCC 4.8+ compatible) static<br>libraries for each architecture and STL variant.<br>_STL variants available:_<br><ul><li>`c++`: LLVM libc++ runtime (recommended)</li><li>`gnustl`: GNU STL</li><li>`stlport`: STLport runtime</li></ul><br>More information can be found in the<br>[NDK C++ Helper Runtimes](https://developer.android.com/ndk/guides/cpp-support.html#runtimes) documentation.
+libs/android/ARCH/STL     | Android (GCC 4.8+ compatible) static<br>libraries for each architecture and STL variant.<br>_STL variants available:_<br><ul><li>`c++`: LLVM libc++ runtime (recommended)</li><li>`gnustl`: GNU STL (deprecated)</li></ul><br>More information can be found in the<br>[NDK C++ Helper Runtimes](https://developer.android.com/ndk/guides/cpp-support.html#runtimes) documentation.
 *Desktop Implementations* |
 libs/darwin               | OS X static libraries (desktop or stub implementations, compiled against libc++)
 frameworks/darwin         | OS X frameworks (desktop or stub implementations, compiled against libc++)