chore: replace usages of obsolete twitch struct with backing field (#3614)

Co-authored-by: Felanbird <41973452+Felanbird@users.noreply.github.com>

src/util/StreamerMode.cpp

@@ -76,7 +76,7 @@ bool isInStreamerMode()
             {
                 shouldShowWarning = false;
 
-                getApp()->twitch2->addGlobalSystemMessage(
+                getApp()->twitch->addGlobalSystemMessage(
                     "Streamer Mode is set to Automatic, but pgrep is missing. "
                     "Install it to fix the issue or set Streamer Mode to "
                     "Enabled or Disabled in the Settings.");