RN: Delete `EventEmitter#removeSubscription`
RN: Delete `EventEmitter#removeSubscription`

Summary:
Deletes `EventEmitter#removeSubscription`, which has been deprecated since D27704279 (https://github.com/facebook/react-native/commit/cb6cbd12f80152b4ce742f37e2e6eefadf89d927).

Relatedly, the `removeListener` family of methods (which were deprecated ad the same time) were recently removed by D35549719 (https://github.com/facebook/react-native/commit/2596b2f6954362d2cd34a1be870810ab90cbb916).

Changelog:
[General][Removed] - Removed `EventEmitter.prototype.removeSubscription` method.

Reviewed By: christophpurrer

Differential Revision: D36171048

fbshipit-source-id: 2409d235d43049cddfe0a54bcc60e1f47d4185c5

javache/react-native,facebook/react-native,javache/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native