x86: remove intr_bind

intr_bind(u_int vector, u_char cpu); looked suspicious since
everywhere else "cpu" is a u_int and >256 processors isn't unreasonable
now. intr_bind() is not used anywhere in FreeBSD (now, after commit
bf42f3738087). Time to remove.

Relnotes: Yes
Reviewed by: mjg
Differential Revision: https://reviews.freebsd.org/D36901