[Android] Check no sync completion callback passed on password removal

The sync completion callback passed to RemoveLoginsByURLAndTime is
only relevant for account passwords which don't exist on Android.

This CL clarifies this point by:
1. Adding comments explaining that the callback is used for account
passwords to ensure the passwords are cleared before the user is
signed out (via cookie removal).
2. Checking that on Android, the callback passed in is null since it isn't relevant.

Bug: 1441112
Change-Id: I2a225693769bc1799f29d4782e27195bec319bf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4496146
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1141302}