Remove DCHECK for null callback in phishing classifier

This DCHECK initially existed because a single page load would request
classification at max once. The CancelPendingClassification function is
called whenever a page is finished loading, so it was expected to be
null hence the DCHECK. With multiple new triggers, a single page load
can have multiple classifications that can occur, so this DCHECK is no
longer applicable.

Bug: 343247631
Change-Id: I35449ec90da6eb5a2f5826dcbab086cbb51e1837
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6105880
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Commit-Queue: Seung Jae Lim <andysjlim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1398237}