Remove invalid reference in docs (#667)

Author: jws

docs/common_issues.rst

@@ -104,7 +104,6 @@ Tracking Custom Users
 
     Use ``register()`` to track changes to the custom user model
     instead of setting ``HistoricalRecords`` on the model directly.
-    See :ref:`register`.
 
     The reason for this, is that unfortunately ``HistoricalRecords``
     cannot be set directly on a swapped user model because of the user

docs/querying_history.rst

@@ -122,9 +122,6 @@ model history.
     <Poll: Poll object as of 2010-10-25 18:04:13.814128>
 
 
-.. _register:
-
-
 Save without a historical record
 --------------------------------