Remove PasswordMetrics#validatePassword()

PasswordMetrics#validatePassword() is no longer used since all its
callers were to converted to use PasswordMetrics#validateCredential()
instead.  So, remove it.

Bug: 219511761
Bug: 232900169
Bug: 243881358
Test: build
Change-Id: Ica7adff4f7171b24518cd916f1d4f6d4707851d6
Merged-In: Ica7adff4f7171b24518cd916f1d4f6d4707851d6
(cherry picked from commit 25053e20)