Remove python v2 remnant

As vermin [0] confirms, the codebase has long moved beyond supporting
python v2 (which is not a bad thing). This removes the last explicit py2
piece of code.

And in case anyone wonders, vermin currently reports the minium version
to be v3.6.

[0] https://pypi.org/project/vermin/

powerdnsadmin/models/user.py

@@ -83,10 +83,7 @@ class User(db.Model):
         return False
 
     def get_id(self):
-        try:
-            return unicode(self.id)  # python 2
-        except NameError:
-            return str(self.id)  # python 3
+        return str(self.id)
 
     def __repr__(self):
         return '<User {0}>'.format(self.username)