Deprecate and remove the Clustering Plugin

[cammace]

Spinning this ticket off of what I mentioned in #518. The Cluster Plugin offers no advantage of GeoJSON runtime clustering and IMO only promotes usage of GLMarkers which don't nearly have the same extensive API set as symbol layers (as we've discussed in great lengths in the past).

cc: @tobrun @zugaldia

[cammace]

After some discussion internally with @tobrun, we have decided the best path forward will be deprecating and removing this plugin. There are several different cases which you might be using clustering for but all the use cases for this plugin can be done using runtime styling as mentioned above. If you have any questions migrating over to runtime styling, please feel free to reach out on this ticket or through our support page.

[zugaldia]

The Demo app contains a few examples of how to implement clustering using runtime styling. Some good reference activities include:

• GeoJsonClusteringActivity
• CreateHotspotsActivity
• SymbolLayerMapillaryActivity

The whole is app open source and available for download in the Play Store.