Disable native source code fetching in jsinspector-modern targets (#46132)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46132

Temporaily disable the `nativeSourceCodeFetching` capability — which reverts this to the legacy handling in the Inspector Proxy.

This is because we've noticed performance issues when loading large bundle source maps, particularly on Android, with a nontrivial path to optimising this ([raising the frontend `IO.read` size](https://github.com/facebookexperimental/rn-chrome-devtools-frontend/pull/97) further is leading to WebSocket disconnections on Android 😐).

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D61543480

fbshipit-source-id: ee66b4cebd40f8cc6466270c5875df744d2b588a