Add removeStar method to Backlog class for star deletion

Author: katayama8000

src/backlog.ts

@@ -874,6 +874,14 @@ export default class Backlog extends Request {
     return this.post('stars', params);
   }
 
+  /**
+   * https://developer.nulab.com/docs/backlog/api/2/remove-star/
+   */
+  public removeStar(starId: number): Promise<void> {
+    const endpoint = `stars/${starId}`;
+    return this.delete(endpoint);
+  }
+
   /**
    * https://developer.nulab.com/docs/backlog/api/2/get-notification/
    */