Revert "[ios] Mark test TestHandleForgottenIdentityPromptSignIn as FLAKY"

This reverts commit 284830b9307aa5ba47a2966c3c52c902d3b176dc.

Reason for revert: Flakiness was fixed by https://crrev.com/c/6494627

Original change's description:
> [ios] Mark test TestHandleForgottenIdentityPromptSignIn as FLAKY
>
> The test is flaky both on simulator and on device.
>
> Bug: 413588295
> Change-Id: I015fc8adb372418e0795cfe9d41486fcc7982761
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6491587
> Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
> Auto-Submit: Federica Germinario <fedegermi@google.com>
> Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1452439}

Fixed: 413588295
Change-Id: I5b84ad2614678ea45258db98140df596815f9f87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6494807
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452622}