power: Remove submenu

The submenu contains a single menu item - a link to Power Settings - that mainly exits as an excuse for the menu itself, which serves as an indicator for the charge-level and time estimate.

However we already have an option for showing the charge-level in the top bar, so revealing it every time the menu is opened can take over the indicator functionality without taking up precious space in the system menu.

Fixes #1241 (closed)