[energy kcm] Remove bogus Has power supply property from UI

Summary:
In a laptop when the ac line is unplugged, kinfocenter wrongly display that the ac power supply is plugged.
See screenshot, "Batterie" section says it is discharging, yet "Système" section reports "Adapteur secteur connect" which translates to "AC power line plugged".

{F7237233}

This is due to "Power Supply" solid property being wrongly interpreted.
This solid property is always true for devices that can deliver power supply, whether or not there active or plugged.
See https://upower.freedesktop.org/docs/Device.html#Device:PowerSupply

Reviewers: broulik, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23130