Bug 1661364 - Part 5: Remove MarkAsTroubled, r=kmag

This method has no implementation, and is unused.

Differential Revision: https://phabricator.services.mozilla.com/D88629

dom/ipc/ContentParent.h

@@ -838,12 +838,6 @@ class ContentParent final
    */
   bool ShouldKeepProcessAlive();
 
-  /**
-   * Mark this ContentParent as "troubled". This means that it is still alive,
-   * but it won't be returned for new tabs in GetNewOrUsedBrowserProcess.
-   */
-  void MarkAsTroubled();
-
   /**
    * Mark this ContentParent as dead for the purposes of Get*().
    * This method is idempotent.