cpu, power: Get rid of the remnants of the EA computation insts.

cpu, power: Get rid of the remnants of the EA computation insts.

Get rid of some remnants of a system which was intended to separate
address computation into its own instruction object.

Change-Id: I23f9ffd70fcb89a8ea5bbb934507fb00da9a0b7f
Reviewed-on: https://gem5-review.googlesource.com/7122
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>