Delete "Compact RelaxNG" / .rnc files

(Follows rP25210 where I deleted actor.rnc)

This deletes the .rnc validation files, upgrading the .rng files to being the source of truth.

• The engine uses, via LibXML2, .rng files to validate XML schemas, to the .rnc files are redundant.
• The RelaxNG "Compact" format is a DSL for the RelaxNG XML format. Its syntax is unique, unlike .rng files which are XML.
• Some errors are likely - I'm not sure anyone has converted the compact files in years.
• The tool to generate .rng from .rnc files is trang (https://github.com/relaxng/jing-trang), which runs on the JVM and is quite annoying to install compared to "your usual text editor".
• The JS components use the full .rng format in XML, so editing this format is already familiar to most people that mod the game.

The .rnc files were added in rP15377 along the .rng files.

Refs #413, #245

Differential Revision: https://code.wildfiregames.com/D3824