Now that the deprecated SoupSessionAsync has been replaced with
SoupSession [1], the response callbacks are invoked in the next
iteration of the main loop after soup_session_abort has returned. This
means that http_client_check_response_cb is no longer called from a
GCancellable::cancelled signal handler. Therefore, it's safe to
directly disconnect from the GCancellable in the response callback
without fearing for any deadlocks.

This reverts commit b2f94098.

[1] Commit 6c3e3c2d

https://bugzilla.gnome.org/show_bug.cgi?id=764157