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

[David Spickett via llvm-commits]

DavidSpickett wrote:

The clang one is 99% the same but not knowing exactly what you wanted from it, I'll leave it to you to decide what to do with this backend.

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