Remove deprecations from coord_categorisation module

The coordinate categorisation module contains several types of deprecation, which were introduced in the 1.4 release. Since these deprecations have been present in the 1.4 and 1.5 releases and the 1.6 release will be cut more than 6 months after the 1.4 release, so it must be time to remove them.

The three things that were deprecated:
* custom season categorisation functions: functionality rolled into the regular versions
* shortname categorisation functions: just wrappers around another functions
* default name arguments: changed to more explicit names

The first three commits correspond to removing the deprecations and adjusting the deprecation test code as little as possible to cope, which I think demonstrates that the deprecations are all removed properly. The last commit removes the deprecation testing altogether since it is no longer required.