core: Fix 'soup_gnome_features_2_26_get_type' is deprecated warning

GNOME 2.26 libsoup's feature was just the GNOME proxy resolver,
which is now provided as default GIO extension point.

More informations can be seen in:
[0]: https://git.gnome.org/browse/libsoup/commit/?id=4ded0924fd82e006f31a8e9089fd297fac15eef9
[1]: https://bugzilla.gnome.org/show_bug.cgi?id=573685

https://bugzilla.gnome.org/show_bug.cgi?id=712565