nx: cleanup config (#8979)

The `schema` field is defined twice, delete the second time that
it is defined.

nx.json

@@ -81,6 +81,5 @@
       "dependsOn": ["^build", "build:contracts"],
       "outputs": ["{projectRoot}/dist"]
     }
-  },
-  "$schema": "./node_modules/nx/schemas/nx-schema.json"
+  }
 }