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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30590