Remove HAVE_LIBPSAPI, HAVE_SHELL32.

These used to be set in the old autoconf build, but the cmake build has had a
"TODO: actually check for these" comment since it was checked in, and they
were set to 1 on mingw unconditionally.  It seems safe to say that they always
exist under mingw, so just remove them and assume they're set exactly when on
mingw (with msvc, we use `pragma comment` instead of linking these via flags).

llvm-svn: 328992