i386: Make setidt_disp a size_t instead of uintptr_t

[markj]

Summary

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 a previous commit, I'm not
sure why I thought it was required.

[jrtc27]

Thanks :)