gunix{input|output}stream: Drop custom close_{async|finish}() methods
They were not actually asynchronous, and hence caused blocking in the main thread. Deleting them means the default implementation of those vfuncs is used, which runs the sync implementation in a thread — which is what is wanted here.
Spotted by Benjamin Otte.
Signed-off-by: Philip Withnall withnall@endlessm.com
Fixes: #2051 (closed)