Clean up db8 upstart scripts

:Release Notes:
Clean up db8 upstart scripts

:Detailed Notes:
- Remove mediadb upstart files
- Remove tempdb upstart files
- Change db8 start condition for fast & stable boot seq.

Here is boot positions
- core-boot-done : very important services for factory line
- datastore-init-start : DB8 and SettingsServices and filecache
- init-boot-done : The services which needed for last-input-app
- minimal-boot-done : The services which needed for app launching
- rest-boot-done : Other services which can be deferred
- boot-done : Boot complete

:Testing Performed:
- #475 + This commit
- Db8 works fine in auto mode.

:QA Notes:

:Issues Addressed:
[GF-55278] workload distribution to improve input responsiveness during boot

Open-webOS-DCO-1.0-Signed-off-by: Sangwoo Kang <sangwoo82.kang@lge.com>

Change-Id: I9b374b948051e4868371a91bfc38e9216cacceff
Reviewed-on: https://g2g.palm.com/4313
Reviewed-by: DCO Verification
Reviewed-by: Sangwoo Kang <sangwoo82.kang@lge.com>
Tested-by: Sangwoo Kang <sangwoo82.kang@lge.com>
Reviewed-by: Ed Chejlava <ed.chejlava@lge.com>
Reviewed-by: Oleksii Uratkin <oleksii.uratkin@lge.com>