Remove unused and undocumented binary schema type

Moved from GitHub dgraph/3328

Posted by danielmai:

What version of Dgraph are you using?

latest, master

Have you tried reproducing the issue with latest release?

Yes

Steps to reproduce the issue (command/config used to run Dgraph).

Alter the schema to set a predicate to a binary type.

Expected behaviour and actual result.

Dgraph no longer needs a binary type exposed as an available predicate type. They were originally sent as bytes from the client and are returned as base64-encoded strings in JSON responses. Everything that’s possible with binary types including custom tokenizers is already available for strings.