arm64: Stop setting VM_BCACHE_SIZE_MAX

This setting places a (small) limit on the size of the buffer cache,
constraining UFS performance on large servers. The setting comes from
the initial arm64 implementation and appears to be vestigal. Remove it.

Reviewed by: kib
Submitted by: Klara, Inc.
Sponsored by: Ampere Computing
Differential Revision: https://reviews.freebsd.org/D28162