fix: revert admin redirect (gatsbyjs#9728)

Resolves issue created in gatsbyjs#9269

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packages/gatsby-plugin-netlify-cms/src/gatsby-node.js

@@ -146,13 +146,3 @@ exports.onCreateWebpackConfig = (
     webpack(config).run()
   }
 }
-
-exports.onPostBuild = ({ actions }, { publicPath = `admin` }) => {
-  const { createRedirect } = actions
-
-  createRedirect({
-    fromPath: `/${publicPath}`,
-    toPath: `/${publicPath}/`,
-    isPermanent: true,
-  })
-}