Remove modules-style IOC packaging

[minijackson]

It turns out we don't use the interesting features of the module system, like imports and merging configuration, so this is a lot of code that doesn't need to exist.

To be done:

• Remove packaging EPICS tops with NixOS-style modules
• Don't set EPNIX_<...> as a varname for the configure/RELEASE.local
• Add a NixOS module that provides systemd services for launching IOCs