[lldb] Delete UnwindMacOSXFrameBackchain

Summary:
It isn't used anywhere (except on imaginary triples like
sparc-apple-ios) and it also violates plugin separation.

This patch deletes it and declares UnwindLLDB to be _the_ lldb unwinder.

Reviewers: jasonmolenda, JDevlieghere, xiaobai

Subscribers: jyknight, mgorny, krytarowski, fedor.sergeev, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D75680