[CC Number Validation Target] Remove card network method (7f/8)

This patch removes the CreditCard::GetCardNetwork() method that was
being used only in iOS settings, which are switched over to using the
GetCardNetwork() method directly from credit_card_number_validation.h
header file.

Design doc: https://bit.ly/cc-number-validation-target-dd

Bug: b:281812289
Change-Id: I701a316331dcfcf8e8db24a8ac6bcdb9ad9b06f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5622034
Reviewed-by: Vidhan Jain <vidhanj@google.com>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1313982}