[libc++][CMake] Remove unnecessary conditional for defining new handlers

It turns out that whether the new handlers should be provided is orthogonal
to whether new/delete are provided in libc++ or libc++abi. The reason why
I initially added this conditional is because of an incorrect understanding
of the path we're taking when building on Apple platforms. In fact, we
always build libc++ on top of libc++abi on Apple platforms, so we take
the branch for `LIBCXX_BUILDING_LIBCXXABI` there.

llvm-svn: 358616