Fix #8109: Don't call fetch promotions on uninitialized rewards servi…

…ces (#8110)

Calling rewards engine APIs before the engine has been initialized can cause a crash because the `WhenReady` method used inside `RewardsEngineImpl` will queue up what is assumed a `SingleThreadedTaskRunner`. This is not the case for iOS which uses a standard sequenced task runner.

This call to fetch promotions happens when showing the rewards panel but the rewards engine may not be initialized until the panel is shown (a sec/privacy requirement to not make API calls to rewards services until a user interacts with the feature), so it was being scheduled instead of executing immediately, thus hitting the threading issue on the brave-core side

Sources/Brave/Frontend/Browser/BrowserViewController/BVC+Rewards.swift

@@ -80,8 +80,6 @@ extension BrowserViewController {
     }
     // Hide the current tab
     rewards.rewardsAPI?.selectedTabId = 0
-    // Fetch new promotions
-    rewards.rewardsAPI?.fetchPromotions(nil)
   }
 
   func claimPendingPromotions() {