Listener: Drop support for @softDepend annotation

literally nobody uses this. I don't think anyone even knows it exists.
It's also an obstacle to separating event handler registration from PluginManager.

src/event/Listener.php

@@ -43,9 +43,6 @@ use pocketmine\plugin\PluginManager;
  * Functions which meet the criteria can have the following annotations in their doc comments:
  *
  * - `@notHandler`: Marks a function as NOT being an event handler. Only needed if the function meets the above criteria.
- * - `@softDepend [PluginName]`: Handler WILL NOT be registered if its event doesn't exist. Useful for soft-depending
- *     on plugin events. Plugin name is optional.
- *     Example: `@softDepend SimpleAuth`
  * - `@handleCancelled`: Cancelled events will STILL invoke this handler.
  * - `@priority <PRIORITY>`: Sets the priority at which this event handler will receive events.
  *     Example: `@priority HIGHEST`