added polyfill for browser.runtime.onMessage.removeListener

lib/browser-polyfill/custom-browser-polyfill.js

@@ -174,7 +174,8 @@
 								});
 							return true;
 						}
-					})
+					}),
+					removeListener: listener => browserAPI.runtime.onMessage.removeListener(listener)
 				},
 				onMessageExternal: {
 					addListener: listener => browserAPI.runtime.onMessageExternal.addListener((message, sender, sendResponse) => {