vm: Remove kernel stack swapping support, part 2

After mi_startup() finishes, thread0 becomes the "swapper", whose
responsibility is to swap threads back in on demand. Now that threads
can't be swapped out, there is no use for this thread. Just sleep
forever once sysinits are finished; thread_exit() doesn't work because
thread0 is allocated statically. The thread could be repurposed if that
would be useful.

Tested by: pho
Reviewed by: alc, imp, kib
Differential Revision: https://reviews.freebsd.org/D46113