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

The early return introduced by 04c982a96e19f2254739c24ee8c6b568edcb4190 prevents parent::tearDown() being called on Oracle.

This patch is ultra low priority as it is merely a precaution in case tearDown() is used later on.

@PVince81 @LukasReschke @nickvergessen