[UPMLocalPwd] Remove sync callback null CHECK from the local backend

The local android password store backend used to CHECK that the sync
callback it gets passed is null. This was a way to clarify for the
callers that the sync callback wouldn't be used, so they should
only pass a null one.

However, the account android backend has started using this callback
and requires it to be not null. The caller,
AndroidBackendWithDoubleDeletion, cannot make the difference between a
local backend and an account backend so a non-null callback now makes it into the local backend too.

=> the CHECK needs to be removed. A comment at the caller level documents the fact that the local backend doesn't use that callback.

Bug: 331517808, 321220529
Change-Id: Iff3aa1d0048e2c4cc54fc4cc8dc59260b75648ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398104


Reviewed-by: Karol Sygiet <sygiet@google.com>
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278874}