flatpak: Drop network requirement
The network access is no more needed for gvfs support as it currently
uses named sockets instead of abstract ones. It is enough to allow access
to xdg-run/gvfsd
folder, where the named sockets reside. I'm not aware
of other reasons for allowing network access. Let's drop the network
access. This also workarounds "Too many open files" errors caused by
leftover sockets (gvfs#542 (closed)).
Relates: gvfs#515 (closed)