cmd/internal/obj/arm64: remove AMOVBU from optab

cmd/internal/obj/arm64: remove AMOVBU from optab

The instruction format of MOVBU is the same with MOVB, this CL deletes
MOVBU from optab for simplicity.

Change-Id: Ib034d6c29dd9793cf3e6f9fa8abff0ed0d931d0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/445295
Run-TryBot: Eric Fang <eric.fang@arm.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>