[llvm] [LLVM] Update AArch64 maintainers (PR #120440)

[Amara Emerson via llvm-commits]

aemerson wrote:

Tim made the decision a few months ago to pursue other interests and I'm not sure if he'll be involved with AArch64. Given that I think it's ok to remove him to make the list accurate w.r.t activity. If he decides to return we can of course re-add.

https://github.com/llvm/llvm-project/pull/120440