box: drop redundant guest password checks from authenticate

The guest can't have a non-empty password - it's explicitly prevented by
the _user trigger callback:

https://github.com/tarantool/tarantool/blob/2ed06651c317f2d46631b0d55e4b88f2c8a6c0a0/src/box/alter.cc#L2930-L2936

So there's no need to check the guest user password in authenticate.

NO_DOC=code cleanup
NO_TEST=code cleanup
NO_CHANGELOG=code cleanup

(cherry picked from commit 26f69561)