Don't require status checks for staging

See #134 (comment)

Author: infinisil

rulesets/nixpkgs/required-status-checks.json

@@ -11,7 +11,6 @@
       "include": [
         "~DEFAULT_BRANCH",
         "refs/heads/release*",
-        "refs/heads/staging*",
         "refs/heads/haskell-updates"
       ]
     }