chore: remove webpack from yarn dependencies in Rails dev

This dependency was originally introduced in PR 33079 in
Rails 6.0 (2018).

Follows up PR 43172, which replaced webpacker 6 with
jsbundling-rails in Rails 7.0 (2021).

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

package.json

@@ -4,8 +4,5 @@
     "actioncable",
     "actiontext",
     "activestorage"
-  ],
-  "dependencies": {
-    "webpack": "^4.17.1"
-  }
+  ]
 }