liblink: remove class from %#D formatter on 6l

This was a copy-paste error from 9l.  Besides incorrectly referring to
cnames9, 6l doesn't even use a->class, so simply remove this.

Fixes #9320

Change-Id: I0e3440c9dae1c3408eb795b3645f9f1dd8f50aed
Reviewed-on: https://go-review.googlesource.com/1516


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