[llvm] [AArch64] Remove the Z#_HI register definitions (PR #66353)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

This wasn't investigated because with the definitions being unused I figured this was the standard noise you sometime get when tablegen defs get reordered. I guess it's possible the fake sub registers were blocking a code path or two.

https://github.com/llvm/llvm-project/pull/66353