Add support for MySQL's mandatory ON clause in the DROP INDEX statement

[lukaseder]

MySQL's DROP INDEX statement has a mandatory ON clause, as index names are not unique:
https://dev.mysql.com/doc/refman/5.7/en/drop-index.html

[frodnix]

Now jooq have the following problem - I can't code database-independent DROP INDEX. I suppose the correct way should be dropIndex(indexName).on(tableName) and jooq should produce ON clause for MySQL and no ON clause for other databases.

[lukaseder]

You're right, thank you very much for the hint. Indeed, we'll do that for jOOQ 3.10. I've created a separate feature request: #6258