bitbake: toaster: do image and artifact scan on BuildCompleted

bitbake: toaster: do image and artifact scan on BuildCompleted

Move the image and artifact scan code from toaster.bbclass and
consolidate its logic with the existing logic in buildinfohelper.

Remove handler setup for events which used to be fired from
toaster.bbclass but which are now handled directly by buildinfohelper.

[YOCTO #8556]

(Bitbake rev: f0085cd554604cfff4a3f40a34825fbb6878004f)

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: bavery <brian.avery@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>