Remove 2.8 branch from patch documentation

Author: Tristan Roussel

contributing/code/patches.rst

@@ -110,8 +110,7 @@ work:
 * ``2.3``, if you are fixing a bug for an existing feature (you may have
   to choose a higher branch if the feature you are fixing was introduced
   in a later version);
-* ``2.8``, if you are adding a new feature which is backward compatible;
-* ``master``, if you are adding a new and backward incompatible feature.
+* ``master``, if you are adding a new feature.
 
 .. note::