remove [github] admin routes (#7105)

config/custom-environment-variables.yml

core/server/secret-is-valid.js

core/server/server.js

core/token-pooling/token-pool.js

core/token-pooling/token-pool.spec.js

scripts/github_token_backup.fish

services/github/auth/admin.js

services/github/auth/admin.spec.js

services/github/github-api-provider.js

services/github/github-constellation.js