remove unused import in Flowable.java (#6470)

Author: ojh102

src/main/java/io/reactivex/Flowable.java

@@ -17,7 +17,6 @@
 
 import org.reactivestreams.*;
 
-import io.reactivex.Observable;
 import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.exceptions.Exceptions;