remove cold start gauge name

Author: tejasd

firebase-perf/src/main/java/com/google/firebase/perf/session/SessionManager.java

@@ -39,9 +39,7 @@ public class SessionManager {
 
   @SuppressLint("StaticFieldLeak")
   private static final SessionManager instance = new SessionManager();
-
-  private static final String COLD_START_GAUGE_NAME = "coldstart";
-
+  
   private final GaugeManager gaugeManager;
   private final AppStateMonitor appStateMonitor;
   private final Set<WeakReference<SessionAwareObject>> clients = new HashSet<>();