Revert "Add debug output temporary"

This reverts commit 71cffbce350d8076b39337f3a1bf83e167ccc291.
Don't zero out the Erlang binary password

password.data is a pointer to the real Erlang binary data, not a copy of
it. So if we zero out this we change the "immutable" password Erlang
binary which rabbit_auth_backend_internal will use for it's
authentication.
After discussions, time skew should be an error
Handle non-existant user as rabbitmq internal does

I.e. by handling it as an refusion rather than an error.