Remove neo4j, protobuf support

[linas]

The neo4j bindings are stale, out-of-date. Rather than dragging them into the future, maybe they should be removed. Opinions? See pull req #2142

Pros:

• Makes atomspace smaller, simpler, easier to understand
• Eliminates pointless historical baggage.
• "The Life-Changing Magic of Tidying up with Marie Kondo"
• Cleanliness, minimal design,

Cons:

• Old work is lost. Well, placed where it becomes hard to find, hard to resuscitate.

[amebel]

I don't have a historic perspective on why protobuf was chosen as compared to other serialization tools. What I can gather is that it was introduced before its use in opencog-neo4j. Then what other use could it have?

When interfacing with the atomspace, over a network, there are two main options of data representation that are used, json (REST api & AtomSpacePublisherModule) and scheme (cogserver, ghost_bridge & dist-atomspace through gearman). This might have been sufficient for the use/demo cases. But, is it advisable to follow these representation pattern going ahead, if atomspace is to be used in a commercial settings? I am assuming in a commercial setting, an atomspace will be ingesting data from various systems and responding to queries from other systems or atomspaces (in line with what will be chosen in #2138).

Pros:

• A well known and maintained IDL which can generate code in multiple industrially popular programming languages, thus possibly making atomspace accessible to a broader developer community.
• If the various blog post that compare the speed of serialization and deserialization, and message size of protobuf with json are anything to go by, then protobuf is a keep. We can abduce that it is the same for scheme, as it is textual similar to json.
• "The Life-Changing Magic of Tidying up with Marie Kondo". Take this with a pinch of "Salt Fat Acid Heat", the networking layer of AtomSpaceSubscriber (the only client of AtomSpacePublisher) and the Atomspace Visualizer(REST) may be replaced by grpc/grpc-web, which consume protobuf.
• Cleanliness, minimal design

Cons:

• Who will maintain it?
• Whatever cons it has when compared with other IDLs.

PS
pros: Version 2.0 of "Nobody ever got fired for choosing IBM". It was made by Google 😛

[linas]

Closing. This was done some months ago.