Remove FHC action since it's composite

[gustavderdrache]

Description

We no longer need to push changes to flakehub-cache-action, since it's a composite action now.

Checklist

• Tested changes against a test repository
• Added or updated relevant documentation (leave unchecked if not applicable)
• (If this PR is for a release) Updated README to point to the new tag (leave unchecked if not applicable)

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