libprotobuf dependency

[springmeyer]

Upcoming mapnik-vector-tile/[email protected] have dropped the libprotobuf/protoc dependency. This means that future Mapnik SDKs will no longer need to build and bundle this dep. I think this is true for OSRM as well.

Let's pause on moving on this for a few weeks however as we may still need to do a few release of the current node-mapnik series (v3.4.x) which will always require this dep.

/cc @BergWerkGIS per your question at mapbox/mapnik-vector-tile#1 (comment)