chore[packages/react-devtools]: remove unused index.js (#30579)

This is unused. See
#30533 (comment).

Ran a fresh build of `react-devtools` via `yarn start` in
`packages/react-devtools` to validate that it is still works with React
Native.

Author: hoxyq

packages/react-devtools/index.js

@@ -15,7 +15,6 @@
     "bin.js",
     "app.html",
     "app.js",
-    "index.js",
     "icons",
     "preload.js"
   ],