vm: Remove kernel stack swapping support, part 3

• Modify PHOLD() to no longer fault in the process.
• Remove _PHOLD_LITE(), which is now the same as _PHOLD(), fix up consumers.
• Remove faultin() and its callees.

Tested by: pho
Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D46114