[LLVM][NFC] remove unused fields

Summary:
Here is the commit introducing the fields
https://github.com/llvm/llvm-project/commit/cf6749e4c091

It dates back from 2006 and was used by AArch64 backend.
There is no more reference to these fields in the whole codebase so I think it's fine.

Reviewers: courbet

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66683

llvm-svn: 369810