special handling for format tag in enums that lead to broken code

[bahkauv70]

Definitions of the following structure lead to broken code in the generator:

"MyEnum": {
        "enum": [
          "foo",
          "bar",
          baz"
        ],
        "format": "enum",
        "type": "string"
      }

The additional format specifier is copied verbatim as the base datatype for the enum, leading to broken code if the type is not a builtin type.
The fix for this is to remove the support for the format specifier in the template. This is is a valid approach, because:

• openapi only supports enum for strings, i.e. the basetype always have to be a string
• the format specifier is open-valued (see here), i.e. the application is generator specific
• the base type for the enum can still be changed using the type attribute (even if no enum values can be defined for types other than string!)
• the change has no effect on the current generated API