Please see the FAQ on this.
Q: Can I remove the Esri attribution from my maps?
A: If you use resources from ArcGIS Online no. If you use our map services, you must keep all logos and other attribution on the maps. See this resource for more details on when you can and can't remove the attribution.
Q: Am I required to show Esri attribution in my app?
A: It depends. Esri requires that when you use an ArcGIS Online basemap, Esri data services, or Esri API technology in your app you must also include Esri attribution. See this resource for more details on when to display Esri attribution.
