Bugfix for MojDbShardManagerTest

:Release Notes:

:Detailed Notes:
- fixed bug delString();
- add missing cursor.close() to prevent crash

:Testing Performed:
tested on build

:QA Notes:

:Issues Addressed:
[GF-38959] MojDbShardEngine test hangs in test_db_core

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <ikuksyuk@luxoft.com>

Change-Id: I136997d51e54790daef052ad83a88acd1ba74124
Reviewed-on: https://g2g.palm.com/3471
Reviewed-by: Build Verification
Reviewed-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Tested-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>