Fix import duplication in sage.rings.all

The import of the `all_sagemath_categories` module triggered exactly the same imports that were already in the `rings.all` file. We thus delete this obsolete file.

Author: tobiasdiez

src/sage/rings/all.py

@@ -12,8 +12,6 @@
 # ****************************************************************************
 from sage.misc.lazy_import import lazy_import
 
-from sage.rings.all__sagemath_categories import *
-
 # Ring base classes
 from sage.rings.ring import (Ring, Field, CommutativeRing, IntegralDomain,
                              PrincipalIdealDomain)

src/sage/rings/all__sagemath_categories.py

@@ -3,7 +3,6 @@ py.install_sources(
   'abc.pxd',
   'algebraic_closure_finite_field.py',
   'all.py',
-  'all__sagemath_categories.py',
   'all__sagemath_objects.py',
   'big_oh.py',
   'cc.py',