qapi: Drop redundant, unclean error_is_set()

qapi: Drop redundant, unclean error_is_set()

do_qmp_dispatch()'s test for qmp_dispatch_check_obj() failure examines
both the return value and the error object.  The latter part is
unclean; it works only when do_qmp_dispatch()'s caller passes a
non-null errp argument.  That's the case, but it's not locally
obvious.  Unclean.

Cleanup would be easy enough, but since the unclean code is also
redundant, let's just drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>