[libunwind] Remove the remote unwinding support

This is unfinished, unused and incomplete. This could be brought back in
the future if there's a desire to build a more complete implementation,
but at the moment it's just bitrotting.

Differential Revision: https://reviews.llvm.org/D57252

llvm-svn: 352965
GitOrigin-RevId: 368c02e3ec44e5418626f46abebcc22a69c7f66d
3 files changed