Remove duplicate definitions

[dr-shorthair]

In some parts of the /def/ duplicate entries for the same term appear.
e.g. http://www.opengis.net/def/nil

This appears to apply particularly to OGC-owned terms, due to a legacy of both /ogc/ and /OGC/ URIs appearing in the database.

I suggest retiring one of these branches - I recommend retiring the /OGC/ terms.
In order to support any legacy users, /OGC/ URIs should redirect to /ogc/ URIs.