Remove legacy CMake targets for libcxx and libcxxabi

We've been meaning to remove those targets for a while, and the fix is
simple enough cause they're all just aliases to other targets.

There's no doubt this commit will break some CI systems, however the
fix is trivial.

Differential Revision: https://reviews.llvm.org/D76104

GitOrigin-RevId: f383fb40b17b6b39b37da588ff3c8a2972c685ad
2 files changed