[iOS] Remove spurious DCHECK in SafeBrowsingTabHelper

SafeBrowsingTabHelper has a DCHECK to verify that the URL
received in HandleMainFrameResponsePolicy matches the most
recent URL seen in ShouldAllowRequest. However, this DCHECK
does not hold when a response is served by a ServiceWorker,
as a ServiceWorker is able to set the URL associated with a
response. A previous attempt at fixing this weakened the DCHECK
to only compare the host of the request and response, but there
are cases where even this weaker check won't hold in practice.

This CL removes this DCHECK.

Bug: 1360959
Change-Id: Id8f74f64b47b22e7d8b66a6bf6ca2f7242d53ba9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3879723


Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Joemer Ramos <joemerramos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1044513}