Remove abandoned `cf.etc` directory

As agreed offline, the etc directory has long not been in use (notably duplicating data now in constants module) and can be removed to tidy the codebase and to prevent packaging getting confused and raising warnings about it, e.g. from a CI job:

[90](https://github.com/NCAS-CMS/cf-python/actions/runs/34459805523/job/102814670829?pr=967#step:12:591)
!!
  check.warn(importable)
/private/var/folders/20/jp1_0n3n7kndh6rnbqb5344m0000gn/T/build-env-27lv9eqx/lib/python3.14/site-packages/setuptools/command/build_py.py:215: _Warning: Package 'cf.etc.coordinate_reference' is absent from the `packages` configuration.
!!

        ********************************************************************************
        ############################
        # Package would be ignored #
        ############################
        Python recognizes 'cf.etc.coordinate_reference' as an importable package[^1],
        but it is absent from setuptools' `packages` configuration.

Locate the abandoned cf.etc directory, including cf.etc.coordinate_reference, and inspect how the package is included in the build configuration. Remove the unused directory, then run the packaging or CI check that currently reports the setuptools warning. Done means the directory is gone and the package build no longer warns about cf.etc.