Drop eselect-settingsd, make openrc-settingsd play nice with systemd

When systemd is only used as device manager, settingsd services
will be enabled through dbus activation and redirected to openrc-settingsd.
However, when systemd is the init system, settingsd services dbus
activation won't be used (because systemd itself provides them).
This makes eselect-settingsd no longer needed.