remove all per repo secrets from gha except feedstock tokens

[beckermr]

cc @jaimergp @h-vetinari

[beckermr]

@jaimergp do we use any per repo secrets for CI run or the other alternative CI services?

[jaimergp]

No, they only use the standard GHA secret (STAGING_BINSTAR) which is enabled for all repos already. The only thing we do per-repo is to enable the org-wide Github App installation for that service (Cirun, Blacksmith, Namespace, etc).