[fidl] Remove untyped channel allow list from public_configs

This removes the untyped channels allow-listing from public_configs.
Allow lists in public_configs cause the allow listing to leak somewhat
unpredictably and aren't goverened by the allow list in //build/cpp.

This fixes some use of untyped channels but also puts the allow listing
in the right places.

Bug: 69585
Change-Id: I748b619e206096428e917d306b80f4c29bbbcb33
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/490765
Reviewed-by: Yifei Teng <yifeit@google.com>
Commit-Queue: Ian McKellar <ianloic@google.com>