Fix remove props warnings in CheckboxGroupInput

[SeanBE]

• Dropping the required flag since a default value is provided when destructuring. Proptypes complains when component is child of a ReferenceArrayInput component.
• Pull out loaded attribute from rest to avoid fieldet from complaining.
  index.js:1375 Warning: Received true for a non-boolean attribute "loaded".

[fzaninotto]

In fact, I think the loaded props should be sanitized before passing props to children. So the right fix should be the opposite: add the loaded prop to the sanitizeRestProps of ReferenceArrayInput, and remove it from the SelectArrayInput.

Would you mind working on that patch?