gradually remove `systemd` USE flags from userland applications

[magiruuvelvet]

Most applications don't actually need systemd and continue to function just fine without it. The USE flag causes some applications to link against libsystemd.so which doesn't seem to be necessary for most things. Detaching from libsystemd.so also improves security, as my systemd is frozen at some old version due to compatibility issues with my userland.

Doing this early on should ease the migration to OpenRC in the future.

Related #17

List of liberated packages and software:

• flatpak (including its dependencies)
• KDE
• PulseAudio client libraries
• PipeWire and WirePlumber
• fcitx
• Xorg and Xwayland
• docker
• ...
• dbus (needs to be done last)
  Note: everything that depends on libdbus-1.so also implicitly depends on libsystemd.so which is kinda gross tbh. Unfortunately the systemd USE flag can't be removed from dbus until systemd is removed from the system.