arm64: Remove VPIPT I-cache support from pmap

The VPIPT I-cache policy has been removed from the architecture.
Remove support for it from pmap. The vmm side was not imported so
calls to pmap_invalidate_vpipt_icache would cause the kernel to branch
to a NULL pointer.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47116