[llvm] [llvm][Comment] removing out-of-date comment, ARM backend no longer uses ScheduleDAGRRList (PR #117803)

[via llvm-commits]

CBSears wrote:

@MatzeB Matthias, this was your 2017 comment in ARMBaseRegisterInfo.cpp 
Can you review and commit this?

https://github.com/llvm/llvm-project/pull/117803