cmd: delete old[5689]a

These are the old assemblers written in C, and now they are
not needed.

Fixes #10510.

Change-Id: Id9337ffc8eccfd93c84b2e23f427fb1a576b543d
Reviewed-on: https://go-review.googlesource.com/12784


Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>