core: prevent failure on concurrent live snapshots

Remove redundant freelock() to prevent SQL DuplicateKeyException
on concurrent creation of live snapshots.

Change-Id: Ie745fb194d6cc21ce115fd05ac8c88e852069b74
Bug-Url: https://bugzilla.redhat.com/877750
Signed-off-by: Daniel Erez <derez@redhat.com>