cmd/compile: remove some lines from PPC64.rules

cmd/compile: remove some lines from PPC64.rules

In CL 429035 Keith suggested removing some
rules that were covered by generic rules. This
follows up on that comment.

Change-Id: I57b6c9ae0cd85f33a0eb2fef8356575d3d7820fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/430417
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>