qemu-img: remove references to GEN_DOCS

Nothing seemingly uses this.
(jcody: commit 77bd1119 even mentions that it appears unused)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>