[Auth] Clean-up after auth refactoring on Mac

After recent refactoring, only one version of the strings is used for
both biometric and non-biometric prompts.

This CL:
1- Removes GetMessageForNonBiometricLoginPrompt() method.
2- Removes the non-biometric strings.
3- Renames GetMessageForBiometricLoginPrompt() to
GetMessageForLoginPrompt()

This is a purely mechanical CL.

Bug: 1371026
Change-Id: I9aaccdc1baa86d0c4e57888297924430f976207c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4306702


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112871}