i386: Make setidt_disp a size_t instead of uintptr_t

setidt_disp is the offset of the ISR trampoline relative to the address
of the routines in exception.s, so uintptr_t is not quite right.

Also remove a bogus declaration I added in commit 18f55c67f7, it is not
required after all.

Reported by: jrtc27
Reviewed by: jrtc27, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit cbe59a6475b6c36fac4073bcfc328099fc873420)