[AArch64] Remove inexistent system register ERXTS_EL1

Summary:
AArch64's system register ERXTS_EL1 is present in the backend as a
component of the Arm Reliability, Availability and Serviceability (RAS)
extension. However, it has been removed from the specification before
its final release.

This patch removes the register.

Reviewers: SjoerdMeijer, DavidSpickett

Reviewed By: DavidSpickett

Subscribers: DavidSpickett, kristof.beyls, hiraditya, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79007