[libunwind] Remove the legacy Unwind_AppleExtras.cpp

Unwind_AppleExtras.cpp contained annotations telling the linker that
some symbols are not available on some very old platforms. However,
those platforms are not supported anymore, so the annotations are not
used.

Why remove this? In addition to cleaning up the code base, this also
removes the possibility of implementing those annotations incorrectly
(which was the case previously), which could lead to important symbols
being hidden when they should have been visible.

Differential Revision: https://reviews.llvm.org/D148445