Remove sv_pagesize, originally introduced with r100384.

In all of the architectures we have today, we always use PAGE_SIZE.
While in theory one could define different things, none of the
current architectures do, even the ones that have transitioned from
32-bit to 64-bit like i386 and arm. Some ancient mips binaries on
other systems used 8k instead of 4k, but we don't support running
those and likely never will due to their age and obscurity.

Reviewed by: imp (who also contributed the commit message)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D19280