kde/plasma: update to 5.15.1
I also used this opportunity to clean up the remaining APKBUILDs.
Side note: the quick settings in the top panel now opens the Plasma Settings applications. With this change the NetworkManager configuration can now only handle Wi-Fi. This isn't a problem on any phone, but in Qemu you'll have to connect to the network via nmtui in a terminal.
Edited by Bart Ribbers