conf: Remove kernel stack swapping support, part 12

Remove the NO_SWAPPING option. There is still some code in
vm_swapout.c, but it relates to RACCT handling. Remove the option and
make compilation of vm_swapout.c conditional on RACCT.

Tested by: pho
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46130