bpo-43137: Revert "webbrowser: Don't run gvfs-open on GNOME" (GH-30417)

gvfs-open was deprecated in 2015 and removed in 2018, but its replacement,
gio(1), is not available in Ubuntu 16.04, which is apparently still
supported by CPython upstream even though it is considered to be EOL by
Ubuntu developers.

Signed-off-by: Simon McVittie <smcv@debian.org>