Remove isObservable checks

Styled-JSS should be able to remove the isObservable check, because jss now removes observables from style object, subscribes to it and sets the value directly.

Search the Styled-JSS source for all isObservable checks and read how style objects are passed to JSS. Confirm that JSS handles observable removal, subscription, and direct value updates as described; done means the redundant checks are removed without changing observable styling behavior.

Written by the indexing model from the issue text.