arm64: Remove an old workaround

This is an old workaround for an instruction that wasn't emulated
correctly in some places. As we now read more ID registers that should
also hit this bug trapping this register, and not the others we can
assume nobody hits this anymore.

Reviewed by: harry.moulton_arm.com, imp, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D50204