Remove $wgAuth usage from wrapOldPasswords.php

Remove $wgAuth usage from wrapOldPasswords.php

AuthPlugin::allowSetLocalPassword() has been deprecated with no
real replacement. (Authentication providers largely describe things
from a client POV, so we can check whether the authentication system
is configured to use passwords but not whether it uses the
user_password field. We could check whether the auth providers include
LocalPasswordAuthenticationProvider but that's rather fragile.)

There does not seem to be much need, either; we can assume the site
admin has at least a basic knowledge of what they are using and would
not run wrapOldPasswords.php if the site wasn't using local passwords.

Bug: T180537
Change-Id: I34f2d8ad0d801b5460b768ad1d52b29b0a1b08db