[PowerPC] [Book-E] Remove obsolete interrupt binding workaround

Remove an old workaround that is no longer necessary since rS343824.

There used to be a problem with FMan interrupts firing on multiple CPUS
at the same time.

This ended up being due to multicast interrupts being unsupported in the
Freescale PIC (so instead of using a selection algorithm, it would do some
unspecified action, such as interrupting multiple cpus at random.)

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D23829