libc++abi: remove the duplicated unwind content

The unwinder has been moved into its own project setup at
http://svn.llvm.org/projects/libunwind/trunk.  This simply removes the now
duplicated content.  This move was previously discussed on llvmdev at [1].

[1] http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-January/081507.html

llvm-svn: 235759