[ReactNative] Log with RCTPerformanceLogger only once per bridge instance

Summary:
Fixes #1809

Even if a bridge has more than one root view, it's not supported by the js side
right now, and it will keep warning that those timespans had already been
recorded.

React/Base/RCTPerformanceLogger.m

@@ -64,6 +64,8 @@ RCT_EXPORT_MODULE()
 
 - (void)sendTimespans
 {
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+
   [_bridge enqueueJSCall:@"PerformanceLogger.addTimespans" args:@[
     RCTPerformanceLoggerOutput(),
     @[