[libc++abi] Remove unistd.h include

This was originally there for the _POSIX_THREADS define, to detect the
presence of pthreads. That went away with the externalized threading
support, so the include can go away too.

config.h is now completely empty. A follow-up commit will remove it
entirely.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@299087 91177308-0d34-0410-b5e6-96231b3b80d8