Revert r291022: x86/intr: allow mutex recursion in intr_remove_handler

This was only needed for Xen, and a better way to deal with this issue has
been found, so this commit can be reverted.

Sponsored by: Citrix Systems R&D
MFC after: 5 days
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D7363