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

[David Spickett via llvm-commits]

DavidSpickett wrote:

This used to be used by lldb - https://reviews.llvm.org/D70610 but not since we moved to llvm's command line parser I think.

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