Remove src/* exports mapping

Summary:
Follows up #57277, going further to remove this entry point mapping entirely — D110911864 further supports that these import paths were never used externally (and in the first case, `src/private/` has always been an explicit name).

**One caveat**

`rn-tester` includes some runtime and Fantom test references to `'react-native/src/private/...'`.

- This happens to continue working because we've not enabled `"exports"` resolution under Flow.
- These can be classified as repo-local — motivation is simplifying/tightening the root `package.json` for open source.

Changelog:
[Internal] - (Covered by #57277)

Reviewed By: rubennorte

Differential Revision: D111034634

Author: huntie

packages/react-native/package.json

@@ -44,10 +44,6 @@
       "default": "./Libraries/*.js"
     },
     "./scripts/*": "./scripts/*",
-    "./src/*": {
-      "types": null,
-      "default": "./src/*.js"
-    },
     "./asset-registry": {
       "types": null,
       "default": "./src/asset-registry.js"