Revert "webauthn: make macOS platform credentials always discoverable"

This reverts commit c39aeb48d21904e4fa534f2a6943136ad7eed30c.

Reason for revert: Suspected culprit of bot failures:
https://ci.chromium.org/ui/p/chromium/builders/ci/Mac11%20Tests/11942/overview
https://ci.chromium.org/ui/p/chromium/builders/ci/Mac12%20Tests/2805/overview

Original change's description:
> webauthn: make macOS platform credentials always discoverable
>
> Currently, macOS platform credentials can be discoverable or
> non-discoverable.  This doesn't align well with the rest of the
> ecosystem, and causes some non-obvious quirks (see crbug.com/1025065).
> Change the authenticator implementation to make all newly created
> credentials discoverable, to align with ecosystem expectations.
> Existing non-discoverable credentials remain non-discoverable.
>
>
> Fixed: 1339989
> Change-Id: I9ce342d781df6a795180cfbc4ffd46ce3331f9bf
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3951429
> Reviewed-by: Adam Langley <agl@chromium.org>
> Commit-Queue: Martin Kreichgauer <martinkr@google.com>
> Cr-Commit-Position: refs/heads/main@{#1058450}

Change-Id: I04be9dab01817c03a97ed7037b3e3cb0b58c6b2f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3949177
Auto-Submit: Takumi Fujimoto <takumif@chromium.org>
Owners-Override: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1058495}