The only remaining place that generated ADATA
Prog was the assembler. Stop, and delete some
now-dead code.

Passes toolstash -cmp.

Change-Id: I26578ff1b4868e98562b44f69d909c083e96f8d5
Reviewed-on: https://go-review.googlesource.com/20646Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>