[Payments] Clean up "ScrollToExpand"

Since Expandable payment handler has been enabled by default, this CL
cleans up all occurrences of "ScrollToExpand" in the code base.

Bug: 1094549
Change-Id: Idbf863db8d08b583188bcf923879e9933b497b8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626418
Auto-Submit: Liquan (Max) Gu <maxlg@chromium.org>
Commit-Queue: Nick Burris <nburris@chromium.org>
Reviewed-by: Nick Burris <nburris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843632}

components/payments/content/android/payment_feature_list.h

@@ -13,7 +13,6 @@ namespace android {
 
 // Android only payment features in alphabetical order:
 extern const base::Feature kAndroidAppPaymentUpdateEvents;
-extern const base::Feature kScrollToExpandPaymentHandler;
 
 }  // namespace android
 }  // namespace payments