Remove long attribute accessor methods

[meisyal]

The current version of staruml-ruby (0.1.4) can generate both long and short attribute accessor methods. Long attribute accessor methods are done by setter and getter methods for attribute and the short accessor attribute is done via attr_accessor or attr_reader.

In the future release, long attribute accessor methods will be removed completely. Attribute accessor preference on extension configuration is removed as well. So, by default, this extension can only generate short accessor attribute as a shortcut to access attributes.

[meisyal]

This feature has been implemented in release 0.1.5.