Simplify arm64's pmap_bootstrap() a bit.

locore constructs an L2 page mapping the kernel and preloaded data
starting a KERNBASE (the same as VM_MIN_KERNEL_ADDRESS on arm64).
initarm() and pmap_bootstrap() use the preloaded metadata to
tell it where it can start allocating from.

pmap_bootstrap() currently iterates over the L2 page to find the last
valid entry, but doesn't do anything with the result. Remove the loop
and zap some now-unused local variables.

MFC after: 2 weeks
Sponsored by: Juniper Networks, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D24559