qapi: Don't suppress doc generation without pragma doc-required

qapi: Don't suppress doc generation without pragma doc-required

Commit bc52d03ff5 "qapi: Make doc comments optional where we don't
need them" made scripts/qapi2texi.py fail[*] unless the schema had
pragma 'doc-required': true.  The stated reason was inability to cope
with incomplete documentation.

When commit fb0bc835e5 "qapi-gen: New common driver for code and doc
generators" folded scripts/qapi2texi.py into scripts/qapi-gen.py, it
turned the failure into silent suppression.

The doc generator can cope with incomplete documentation now.  I don't
know since when, or what the problem was, or even whether it ever
existed.

Drop the silent suppression.

[*] The fail part was broken, fixed in commit e8ba07ea9a.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191018074345.24034-2-armbru@redhat.com>