libqtest: Remove qtest_qmp_discard_response() & friends

libqtest: Remove qtest_qmp_discard_response() & friends

qtest_qmp_discard_response(...) is shorthand for
qobject_unref(qtest_qmp(...), except it's not actually shorter.
Moreover, the presence of these functions encourage sloppy testing.
Remove them from libqtest.  Add them as macros to the tests that use
them, with a TODO comment asking for cleanup.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-5-armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>