Don't enable interrupts in init_secondary().

The MI kernel assumes that interrupts will not be enabled on APs until
after the first context switch. In particular, the problem was causing
occasional deadlocks during boot.

Remove an unneeded intr_disable() added in r335005.

Reviewed by: jhb (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18738