cmd/internal/obj: eliminate AMODE

cmd/internal/obj: eliminate AMODE

AMODE appears to have been intended to allow
a Prog to switch between 16 (!), 32, or 64 bit x86.
It is unused anywhere in the tree.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ic57b257cfe580f29dad81d97e4193bf3c330c598
Reviewed-on: https://go-review.googlesource.com/38445
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Cherry Zhang <cherryyz@google.com>