Revert "tests: Add a way to test cancelling introspection gets"

This reverts commit c90f519475e3c710128454188f5c3e5c3ad62ee0.
Revert "Add gupnp_service_info_get_introspection_async_full()"

This reverts commit 909e721c52a335aa8b0ef8f3c37fa62216451530.