Revert "Added support for configuring and obtaining custom implementations of DbSupport from flyway properties such as "flyway.dbsupport.H2=org.myorg.MyH2DbSupport".  This allows organizations to do things such as implement new databases or override methods such as 'clean'.  The 'clean' case is actually the driving force for my work, because we have had repeated issues with Flyway due to it not supporting all objects in all vendor databases.  Rather than asking Axel to support all possible object types, I would prefer to simply add behavior to clean them myself, much as I can add behavior to establish objects of any type in my migration scripts."

This reverts commit b8eaa741d08a209058d81f8ca25a89705f6a7f93.