[libc++] Remove the legacy LIBCXX_LIBCPPABI_VERSION option

I think this notion of libc++abi's version was relevant a long time ago
on Apple platforms when we were using a Xcode project to build the library.
As part of moving Apple's build to CMake, D59489 made it possible to
specify the "ABI version" of libc++abi in use. However, it's not possible
to build libc++abi with that old ABI anymore and we don't need the ability
to link against that version from libc++ anymore.

Hence, we can clean this up and stop falsely pretending that libc++abi
has more than one ABI version.

Differential Revision: https://reviews.llvm.org/D125687