Go all-in on per-map sky clusters files

[res2k]

Per-map sky clusters files are the more flexible approach compared to having a monolithic sky_clusters.txt file.
While Q2RTX does support them, it's not very evident - you need to look at the code to realize this.
To provide more visibility the monolithic sky_clusters.txt was split up into map-specific files (using a script, also included).
The documentation referring to sky_clusters.txt has also been updated to mention per-map sky clusters files instead.
Lastly, I removed the code to read sky_clusters.txt.

(That last one is perhaps debatable... it's certainly not needed and making per-map sky clusters files support more visible/advertised could be enough. However, if the file wouldn't be shipped any more, there's little reason left to keep supporting it in the code, IMO.)