[Password Manager] Remove the "manager-for-sync-signin" flags.

The 'disable-manager-for-sync-signin' and 'enable-manager-for-sync-signin' flags
are not effective, Chrome will never fill in GAIA credentials in the Chrome
sign-in page, because of origin mismatch. This patch is a to address this
cleanup.

BUG=586107

Review URL: https://codereview.chromium.org/1744513002

Cr-Commit-Position: refs/heads/master@{#377889}
4 files changed