Remove dead code in i386 cpu_throw().

Curpmap must be already valid when cpu_throw() is called, even for early
AP startup.

Suggested by: alc
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
Approved by: re (marius)
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D16893