Properly call parent::tearDown() in MDB2SchemaManager::tearDown().

The early return introduced by 04c982a96e19f2254739c24ee8c6b568edcb4190
prevents parent::tearDown() being called on Oracle.
Merge pull request #12981 from owncloud/MDB2SchemaManager-tearDown-Oracle

Properly call parent::tearDown() in MDB2SchemaManager::tearDown().