main/postmarketos-base-ui-plasma: remove unnecessary override files, and move kscreenlocker override back to plasma-mobile

A bunch of override files in postmarketos-base-ui-plasma seem to have come from the plasma-mobile ui package, which probably shouldn't be on desktop and bigscreen.

With Plasma 6, a lot of the overrides are now also no longer necessary for Plasma Mobile.

Situation with each file:

• 000-gschema.override - This likely was for changing maliit themes and GTK title bar layout for Plasma Mobile, maliit theming was dropped (https://github.com/maliit/keyboard/pull/108), and title bar layout is now handled within Plasma Mobile
• kdeglobals - This was for setting the look-and-feel on Plasma Mobile and default browser, it's now handled within Plasma Mobile envmanager
• kwinrc - These were settings for disabling window decorations and setting the vkbd to maliit, which is now handled in Plasma Mobile envmanager
• kscreenlockerrc - This is to autolock the screen after login (tinydm autologs in), the file was for Plasma Mobile, I don't think we want it for Desktop since SDDM is used there