GInputStream: fix default g_input_stream_skip_async() logic

g_input_stream_real_skip_async() wants to use read_async() normally,
but will use skip() in a thread instead if it sees that read_async()
will end up using threads. Except that the test for "will read_async()
use threads" never got updated to know about the GPollableInputStream
support in read_async(), so it was doing the wrong thing in that case.
Fix.

Also remove a small bit of pre-GTask cruft noticed nearby.

https://bugzilla.gnome.org/show_bug.cgi?id=691489
1 file changed