[llvm] [llvm][TableGen] Remove gen-opt-rst backend (PR #71374)

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

@bogner pinged me about this a while ago. I wrote the emitter to generate the initial pages for `lldb` and `dsymutil` but never got around to automating that. If the new clang variant supports those use cases I'm fine with that, otherwise I'd still like to get to that.

https://github.com/llvm/llvm-project/pull/71374