drop support for x86_64-darwin

[dermetfan]

• Nixpkgs expects to drop support for intel macs in 26.11 as per Nixpkgs 25.11 release notes.
• It was decided to no longer build x86_64-darwin on ci.iog.io to reduce strain on the struggling mac builders.

Checklist

• My changes generate no new warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

[coot]

LGTM