Cleanup CreateCluster tool

If the database exists, it gets deleted. If the database doesn't exist, an exception gets thrown, immediately caught, and ignored. In both cases exists will be false and we continue. Later the database
gets deleted unconditionally, which means we can delete this entire block.

that I see on travis CI

Oracle doesn't support enum types

I'm not sure why there is a reference to Oracle in the discussion of https://github.com/h2database/h2database/pull/451. Oracle does not support an enum type (although it can be emulated, as in any database, using a `CHECK` constraint).

* Renamed enum NULL_CONSTRAINT to NullConstraintType
* Replaced some switches with if-statement
* Check for Oracle-Mode when parsing Oracles-SQLs NOT NULL ENABLE
VALIDATE syntax. NOTE: 
* Added DbTypeEnum-class to Mode to have a String-free dbType check
featuring the method Database.isDbTypeOneOf(DbTypeEnum... dbTypes)

Oracles NOT NULL syntax can have optional ENABLE VALIDATE added e.g  
  create table T (C int NOT NULL ENABLE VALIDATE)
Some SQL-generators produce such SQL and H2 can now parse it with this
change.

Issue#647 Removed and ignored generated file.

Issue#628 NPE in CTE