[Sharing] Clear authorized_key field from sharing prefs

The field was used for Vapid messages which is not used anymore.

This CL also stops registration for Vapid on iOS (it wasn't used there
and hence it's safe to remove).

Bug: 40253551
Change-Id: If9467a95edc8b723e15d8bdeea661cf0bc38b9d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6587112
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/main@{#1465800}