arm64: Remove kernel undef instruction support

Now we don't handle any undefined instructions in the kernel remove
the support to handle them. The kernel should only ever execute valid
instructions. The only case it needed to handle was an old emulator
that is now well out of support, and the handler for this has been
removed.

Reviewed by: harry.moulton_arm.com
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D50205