Remove handleDrushParams mentions everywhere

Issue #2938297 on drupal.org by drugan.

Not sure if I am right there but it least I've got confused while exploring the module code. The README.txt states that the "handleDrushParams" method should be implemented and in the DevelGenerateBase this method is implemented but in the DevelGenerateBaseInterface there is "validateDrushParams" method which is implemented by all the DevelGenerate plugin instances.

Needs to be clarified.