[clang] [flang] [flang] Remove legacy (non-HLFIR) lowering and related options (PR #196205)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu commented:

I mainly looked at the driver, frontend, and bbc, and those LGTM. 

https://github.com/llvm/llvm-project/pull/196205