[iOS] Remove sync-trusted-vault-passphrase-ios-rpc flag meta data

This flag meta data should have been removed with crrev.com/c/4011338.

Change-Id: I69ce8e8b47b180e93013dd8ae1e23279a5ff5582
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4028556


Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071567}