block: Don't bother asserting type of output visitor's output

After a visit of a complex QAPI type FOO

    ov = qobject_output_visitor_new(&foo);
    visit_type_FOO(ov, NULL, expr, &error_abort);
    visit_complete(ov, &foo);

we can safely assume qobject_type(foo) is QTYPE_QDICT.  We do in many
places, but occasionally assert qobject_type(obj) == QTYPE_QDICT.
Don't.  The appropriate place to check such fundamental properties of
QAPI visitors is the test suite.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-15-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
