mi_switch.9: Remove cpu_switch, cpu_throw

cpu_machdep.9 was added to document cpu_*, but cpu_switch and cpu_throw
were already documented in mi_switch.9, and MLINKed. cpu_machdep.9
seems like the correct place for this, so remove them from mi_switch.9.

Some of the removed text was stale, although there are few notes that
ought to be added to cpu_machdep.9 in a future commit.

Reported by: tools/pkgbase/metalog_reader.lua
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Fixes: 9c87cbbcaaed ("cpu_machdep.9: New manpage describing the semantics of several cpu_*")
Differential Revision: https://reviews.freebsd.org/D48360