Remove unused IOSChromeSavePasswordInfoBarDelegate::set_handler()

Plus styleguide/readability adjustments in the class, including
a new DCHECK to document a precondition for user_email.

Bug: 1382459, 1392708
Change-Id: I2b700be45e42e480df8d5a6abee91801f0c5d770
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4081181


Reviewed-by: Maria Kazinova <kazinova@google.com>
Commit-Queue: Maria Kazinova <kazinova@google.com>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1081426}