[flang][fir] Small clean-up in `fir_DoConcurrentLoopOp`'s defintion (#146028)
Re-organizes the op definition a little bit and removes a method that
does not add much value to the API.
PR stack:
- https://github.com/llvm/llvm-project/pull/145837
- https://github.com/llvm/llvm-project/pull/146025
- https://github.com/llvm/llvm-project/pull/146028 (this one)
- https://github.com/llvm/llvm-project/pull/146033