cmd/internal/obj/arm64: remove unncessary class check in addpool

The argument class check in addpool is unnecessary, remove it so that we don't
need to list all the compatiable classes.

Change-Id: I36f6594db35e25db22fe898273e024c2db4cb771
Reviewed-on: https://go-review.googlesource.com/c/go/+/283492


Trust: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>