Remove AppleClang-5.1-features.rst release notes.

It is redundant after Xcode-clang-compile-features.rst.

Help/release/dev/AppleClang-5.1-features.rst

@@ -1,5 +0,0 @@
-AppleClang-5.1-features
------------------------
-
-* The :manual:`Compile Features <cmake-compile-features(7)>` functionality
-  is now aware of features supported by Apple Clang 5.1 (``AppleClang``).