[clangd] Remove no-op crash handler, we never set a crash context.

Summary:
I think this was just copied from somewhere with the belief that it actually
did some crash handling.

Of course the question arises: *should* we set one? I don't think so:
 - clangd used to crash a lot, now it's pretty stable, because we found and
   fixed the crashes. I think the long-term effects of crashing hard are good.
 - the implementation can't do any magic, it just uses longjmp to return without
   running any destructors by default. This is unsafe in general (e.g. mutexes
   won't unlock) and will certainly end up leaking memory. Whatever UB caused
   the crash may still stomp all over global state, etc.

I think there's an argument for isolating the background indexer (autoindex)
because it's not directly under the user's control, the crash surface is larger,
and it doesn't particularly need to interact with the rest of clangd.
But there, fork() and communicate through the FS is safer.

Reviewers: ioeric, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53034

llvm-svn: 344245