[iOS][MIM2] Remove unused AuthenticationFlowDelegate protocol

This protocol was used the owner of AuthenticationFlow as blocking
the window instead of view.
When AuthenticationFlow needed to display a dialog, the window had to
be unblocked so the user could validate the dialog.

AuthenticationFlow.delegate's methods are not called anymore by
AuthenticationFlow.

AuthenticationFlow is used by AccountMenuCoordinator,
ConsistencyPromoSigninCoordinator, InstantSigninCoordinator and
SigninScreenCoordinator.
None of them are block the window anymore.

For those 2 reasons, AuthenticationFlowDelegate is not needed anymore.

Bug: 375604649
Change-Id: Idc4b8b8737f188eb2c732e544ac69ddd79263a8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6253694
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1419240}