[lldb] Delete dead StackFrameList::Merge

That code is unused since it's check-in in 2010 (and I believe it would leak
memory when called as it releases the passed unique_ptr), so let's delete it.

Reviewed By: vsk

Differential Revision: https://reviews.llvm.org/D100212

GitOrigin-RevId: 5a5a94ed34b07079046ac81e7e97d980ce2c834f
2 files changed