[Signin] Clean up code after Sync Promo for Signin Intercept launch

This CL removes the features kSyncPromoAfterSigninIntercept and
kSigninInterceptBubbleV2 and cleans up the related code, following the
launch of the Sync Promo after Signin Intercept.

The clean up of the local profile creation in the Profile Picker will be
done in a separate CL, as well as a refactoring of the code related to
the previous Profile Customization bubble.

Bug: 1459176
Change-Id: I307f04c65551eeeeb02eff57ed1fe27a36e1710d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4649388
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Gabriel Oliveira <gabolvr@google.com>
Cr-Commit-Position: refs/heads/main@{#1164605}