- fix dropTable() and introduce tableExists()
- kill replaceDB() - this function is unused and it's implementation obviously wrong
- add method annotation OC_DB_StatementWrapper::fetchAll
- remove duplicate code in Test_DBSchema and reuse OC_DB::tableExists
- remove unused variables
Merge pull request #8800 from owncloud/db-changes-as-required-by-8483

Database code cleanup