arm64: Remove ATTR_DEFAULT from pte.h

ATTR_SH(ATTR_SH_IS) will soon be dynamic as the field is moved out of
the page tables in FEAT_LPA2. When this happens ATTR_DEFAULT will just
be ATTR_AF.

Rather than keeping ATTR_DEFAULT with one attribute remove it.

Reviewed by: alc, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46466