core: Remove explicit Plymouth integration

core: Remove explicit Plymouth integration

Even if plymouth is running, it might have not displayed the splash yet,
so we'll see a few lines on fbcon when we should have otherwise had
nothing.

Plymouth integration was added to systemd in commit
6faa11140bf776cdaeb8d22d01816e6e48296971. That same day, Plymouth got
systemd integration [0]. As such, the Plymouth integration has always
been obsolete, and was probably only for older Plymouth's. But I can't
imagine anybody running a Plymouth from 2011 with a systemd from 2015.

Remove the Plymouth/systemd integration, and let Plymouth's code tell
systemd to print the details.

[0] http://cgit.freedesktop.org/plymouth/commit/?id=537c16422cd49f1beeaab1ad39846a00018faec1

Signed-off-by: Jasper St. Pierre <jstpierre@mecheye.net>
Cc: Daniel Drake <dsd@endlessm.com>
Cc: Ray Strode <rstrode@redhat.com>