[AF] Remove iOS WebView PaymentsInterface reliance on VCN flag

Initially brought up in this comment and then discussed over chat where
the consensus was that removing the check is safe:
https://chromium-review.googlesource.com/c/chromium/src/+/7206766/2/components/autofill/core/browser/payments/virtual_card_enrollment_manager.cc#452

Bug: none
Change-Id: I5478f0e9900c1162757539d948109235fdf4d98c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7254644
Commit-Queue: Olivia Saul <osaul@google.com>
Reviewed-by: Carlos Acosta <cyacosta@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1558283}