GSocketConnection: don't close the socket if it's still reffed

When disposing a GSocketConnection, don't explicitly close the
underlying GSocket. The GSocket will close itself if it gets
destroyed, and if it doesn't get destroyed, that presumably means the
app still wants to use it. Eg, this lets you use GSocketClient to
create a GSocketConnection, and then take the GSocket and destroy the
GSocketConnection.

https://bugzilla.gnome.org/show_bug.cgi?id=616855