Use PasswordSaveManagerImpl instead of MultiStorePasswordSaveManager

After recent cleanups, the two classes were functionally identical.
So this CL updates all usages to PasswordSaveManagerImpl (the base
class), and deletes the now-unused MultiStorePasswordSaveManager.

Bug: 1108738
Change-Id: I44ab5f103712519a1ec9e5cacfcda328571e7225
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3142429


Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918737}