Remove create[T](f: Observer[T] => Subscription)

[samuelgruetter]

Let's remove

def create[T](f: Observer[T] => Subscription): Observable[T]

once the deprecation process/release schedule allows it.
Reason: It was removed in RxJava, and people get confused by its presence, eg:

• http://stackoverflow.com/questions/39212913/subscription-without-any-parameters
• Improve implicit convertion #195

[samuelgruetter]

Note: We already agreed to deprecate it in #196

[zsxwing]

👍 I saw no course for RxScala now: https://www.coursera.org/specializations/scala

[dhoepelman]

Due to project EOL status, this will not be removed