core: LiveSnapshotTaskHandler - remove setExecutionContext(null)

LiveSnapshotTaskHandler -> endSuccessfully / endWithFailure:
remove 'enclosingCommand.setExecutionContext(null)'
to avoid an NPE on CommandBase -> buildCommandEntity.

Change-Id: I45ee451b62ff1f41f8ad03f707c105c1db70a768
Bug-Url: https://bugzilla.redhat.com/1147971
Signed-off-by: Daniel Erez <derez@redhat.com>