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. (== for renice jobs in boottime)

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

:QA Notes:

:Issues Addressed:
[GF-44168] [Boottime] Optimize boot sequence

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

Change-Id: I4d6dfe385931ed3b408eb1c09714fc3b3f1efe88
Reviewed-on: https://g2g.palm.com/3947
Reviewed-by: DCO Verification
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>