Remove password_length default docs in config template

The default specified in the docs does not match up with the default
specified in the config.

See
https://github.com/plataformatec/devise/blob/cc8636cfed2801db00bf38f335a3bcc5c79660d3/lib/devise.rb#L127

Changing the docs to read 6..128 with the config setting to 8..128 is
feared to cause confusion, so removing the default clause instead.
Merge pull request #2868 from rmm5t/patch-1

Update password_length docs in config template