Correct command meta alias removal behavior and add appropriate unit tests.

Apparently @hugmanrique caught the issue but suggested the wrong fix. This is the correct fix, and respects the Javadoc.

api/src/main/java/com/velocitypowered/api/command/CommandManager.java

@@ -75,6 +75,13 @@ public interface CommandManager {
    */
   void unregister(String alias);
 
+  /**
+   * Unregisters the specified command from the manager, if registered.
+   *
+   * @param meta the command to unregister
+   */
+  void unregister(CommandMeta meta);
+
   /**
    * Retrieves the {@link CommandMeta} from the specified command alias, if registered.
    * @param alias the command alias to lookup