EmPy: Drop support

[oliver-sanders]

Cylc has provided EmPy support for a few years, however, it has had very little uptake in that time.

• EmPy support was provided at the request of a project which has since moved away from it.
• Support has been broken since November 2023, however, this has gone unreported until recently.
• It would appear that no Cylc users are making use of EmPy support (they can't be, it's broken).
• Cylc-EmPy integration is poorly developed in contrast to Jinja2 support.
• EmPy is a high risk dependency (single maintainer, no public source repo, intermittent release cycle).

As a result we have agreed to retire support (meeting 2024-11-04).

Cylc's preprocessor logic is fairly well abstracted (shebang to identify preprocessor, relatively simple to write a preprocessor module, Cylc&Rose both abstract template variable arguments from the preprocessor engine) so supporting other preprocessors remains possible, though probably something we would want to confine to a plugin rather than accepting into the core repository. We should aim to abstract the pre-processor interface to make this more pluggable in the future.

TODO:

• Cylc Flow (remove references, add helpful error message)
• Cylc Rose (remove references)
• Docs (remove references)
• Syntax files (we can remove references here, but don't need to).