Revert "[PRImpl] Assert mMerchantSupportsAutofillCards != null"

This reverts commit 72c295444f0c29500abe40df542b042209f6ea15.

Original CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2388561

Reason for revert:
This was landed after we reverted its dependent CL, causing assert
failure:
https://luci-logdog.appspot.com/logs/chromium/android/swarming/logcats/4e6e16a4bd138411/+/logcat_logcat_6189963507448870930_20200903T185337-UTC_0cbc662c032fd958

Bug: 1107039
No-Try: true
Change-Id: I8485665142c4ea446406cd9261756b1dd51f49de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393061
Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Haiyang Pan <hypan@google.com>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804406}