[Extensions] Remove 'internal' extension IDL extended attribute parsing

Support for the 'internal' extended attribute for IDL in the json schema
compiler was added back in:
 https://chromiumcodereview.appspot.com/10677015

However it is no longer used anywhere except for the tests, so it looks
like we can remove it.

Note: Lack of usage was verfied by searching codesearch for any
generated schema files with `"internal":true`:
https://source.chromium.org/search?q=f:out%2F%20%22%5C%22internal%5C%22:true%22

No behavior change is expected.

Bug: 340297705
Change-Id: Ia2cceb9ac23bcda0f1053ee9db217807225d0536
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5807288
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345745}