[Autofill] Merge content_autofill_features.{h,cc} with autofill_features.{h,cc}

The argument for introducing content_autofill_features.{h,cc} was
a "conceptual layering violation" [1]. That's a fair point, but the
same argument applies to many other existing features (e.g., some
payments features). Autofill's current practice is apparently to
to organize the features more by team than by architecture.

Therefore, this CL moves `kAutofillSharedStorageServerCardData` to
autofill_payments_features.{cc,h}.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/5199136/comment/e12b2128_05fb46c4/

Bug: 40100455
Change-Id: If938161b1e66074f4585cf6a8dbb9253a1069a41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6039765
Commit-Queue: Christoph Schwering <schwering@google.com>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1386838}