[search_engines] Further narrow CHECK in TemplateURLService::Remove()

We added a CHECK to make sure users aren't removing the Default Search
Engine. But users in the wild DO indeed have multiple engines sharing
the same Sync GUID.

We've encountered an Extensions bug that spawns these, and given the
widespread hitting of this CHECK, there's probably other reasons why
users are getting these.

This CL weakens the CHECK to prevent excess crashing.

Bug: 1166372
Change-Id: Ieeeca67da5224c9331e574bea527db033677df34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2737787
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#860292}