Migrate /clear command to Helix API (#3994)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

tests/src/HighlightController.cpp

@@ -217,6 +217,14 @@ public:
                       failureCallback)),
                 (override));
 
+    // The extra parenthesis around the failure callback is because its type contains a comma
+    MOCK_METHOD(void, deleteChatMessages,
+                (QString broadcasterID, QString moderatorID, QString messageID,
+                 ResultCallback<> successCallback,
+                 (FailureCallback<HelixDeleteChatMessagesError, QString>
+                      failureCallback)),
+                (override));
+
     MOCK_METHOD(void, update, (QString clientId, QString oauthToken),
                 (override));
 };