network: Remove overflow implementation

network: Remove overflow implementation

The code is complicated by requiring overflow, and in order to incrementally
improve the code to match the designs, remove overflow.

In the new design, we'll have a fixed number of menu items, and Wi-Fi
will be done by a separate design, so we can't be too concerned with
the menu not fitting on the screen.

This is a part of the new system status design, see
https://wiki.gnome.org/GnomeShell/Design/Guidelines/SystemStatus/
for design details.

https://bugzilla.gnome.org/show_bug.cgi?id=704670