Remove deprecated functions and widgets

GNOME Settings still has a considerable number of deprecated functions being used right now. It is not good for the health of the code base to have such a high number of of deprecated functions.

This is also a blocker for GTK4 migration.

Current problems

• Deprecated functions are being used.

Goals & use cases

• Cleanup the code base.
• Make the code base more maintainable.
• Prevent potential bugs by removing dead code.

Requirements

• No deprecated functions are still used.

Relevant art

N/A

Proposal & plan

• Printers (@mkasik and/or @felipeborges)
• User Accounts (@oholy and/or @felipeborges)
• Mouse & Touchpad (@felipeborges)
• Wacom (@carlosg)