Docs: Remove optional parameter for removeAllListeners.

Fixes #9277

docs/api/ipc-main.md

@@ -78,9 +78,9 @@ Removes the specified `listener` from the listener array for the specified
 
 ### `ipcMain.removeAllListeners([channel])`
 
-* `channel` String (optional)
+* `channel` String
 
-Removes all listeners, or those of the specified `channel`.
+Removes listeners of the specified `channel`.
 
 ## Event object