src/sage/features/sagemath.py: remove xrefs to sagemath_foo SPKGs

These no longer exist, so the xrefs break the docbuild.

Author: orlitzky

src/sage/features/sagemath.py

@@ -1,32 +1,18 @@
 r"""
 Features for testing the presence of Python modules in the Sage library
 
-All of these features are present in a monolithic installation of the Sage library,
-such as the one made by the SageMath distribution.
-
-The features are defined for the purpose of separately testing modularized
-distributions such as :ref:`sagemath-categories <spkg_sagemath_categories>`
-and :ref:`sagemath-repl <spkg_sagemath_repl>`.
+All of these features are present in a monolithic installation of the
+Sage library, such as the one made by the SageMath distribution.
 
 Often, doctests in a module of the Sage library illustrate the
-interplay with a range of different objects; this is a form of integration testing.
-These objects may come from modules shipped in
-other distributions. For example, :mod:`sage.structure.element`
-(shipped by :ref:`sagemath-objects <spkg_sagemath_objects>`,
-one of the most fundamental distributions) contains the
-doctest::
-
-    sage: G = SymmetricGroup(4)                                                         # needs sage.groups
-    sage: g = G([2, 3, 4, 1])                                                           # needs sage.groups
-    sage: g.powers(4)                                                                   # needs sage.groups
+interplay with a range of different objects; this is a form of
+integration testing::
+
+    sage: G = SymmetricGroup(4)
+    sage: g = G([2, 3, 4, 1])
+    sage: g.powers(4)
     [(), (1,2,3,4), (1,3)(2,4), (1,4,3,2)]
 
-This test cannot pass when the distribution :ref:`sagemath-objects <spkg_sagemath_objects>`
-is tested separately (in a virtual environment): In this situation,
-:class:`SymmetricGroup` is not defined anywhere (and thus not present
-in the top-level namespace).
-Hence, we conditionalize this doctest on the presence of the feature
-:class:`sage.groups <sage__groups>`.
 """
 
 # *****************************************************************************