Drop '/optimade' from paths in openapi.json

[merkys]

Path /optimade is not required in the URLs by the specification, thus I assume it should be dropped from the schema.

[CasperWA]

Agreed. This has already been discussed in another issue or PR, brought up by @ltalirz - we both agree on it and it's good to have it as a specific issue here.
Edit: It's issue #193.

What we couldn't agree on (or rather, what was more uncertain) was what to do with the versioned URL part, i.e., /v0 or /v1.
Personally, I would like to keep it in, but I also see @ltalirz's point that the version is already stated in the OpenAPI, and so it is essentially redundant - however, since this is a required part of the URL, I think it better to leave it in, explicitly.

[merkys]

I would vote for keeping the version URL part, but either way is fine with me for now.

[ltalirz]

Then let's keep the /v0 in the openapi.json for the moment.