Method AbstractSchemaManager->dropAndCreateSequence() contains erroneous code.

I needed to use the function and looking at the code I found that it seemed partially implemented, cut and pasted from something else or just not ported from an earlier implementation. I'm now submitting a patch that fixes this issue. I didn't modify the AbstractSchemaManager->dropSequence($name) signature to become AbstractSchemaManager->dropSequence($sequence) but it might be a good idea to refactor it (guess that would be another issue though )