[password manager] Plumb removing passkeys

Plumb removing passkeys from the front-end to the PasskeyModel.
Rename `passwordsPrivate.removeSavedPassword` to `removeCredential`.

Tapping "Delete" is enough to trigger removing a passkey. A follow-up
will add a modal dialog confirmation.

For now, SavedPasswordsPresenter::RemoveCredential triggers a login
changed event. This will be replaced by an observer interface.

This feature is guarded by the WebAuthenticationManageGPMPasskeys flag.

Design doc:
https://docs.google.com/document/d/1Ox_VBGNO-Ff9bcP97FUMcN15Xtfn_sz0_WuDe3ujbwA

Low-Coverage-Reason: defaulted dtor that is never called.
Bug: 1432717
Change-Id: I735b4c7a916bbb34fb70716da65dd0367b7b65a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4568555


Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1153880}