Remove NASAGIBS Blue Marble example

[Conengmo]

https://github.com/python-visualization/folium/pull/1882 was very helpful in showing that the currently listed example tileset doesn't work anymore. But the proposed replacement is not suitable unfortunately. Simply solve this issue by removing the example. If users want more examples, they can visit the site we mention: http://leaflet-extras.github.io/leaflet-providers/preview/