Remove environment variables page from Turbopack docs (#7929)

### Description

This is not fully correct. Removing it.

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docs/pages/pack/docs/features/_meta.json

@@ -5,6 +5,5 @@
   "css": "CSS",
   "dev-server": "Dev Server",
   "static-assets": "Static Assets",
-  "imports": "Imports",
-  "environment-variables": "Environment Variables"
+  "imports": "Imports"
 }