[2.0.x] Simplified GeometryField.select_format().

Backport of e9a370bb6a from master

django/contrib/gis/db/backends/base/operations.py

@@ -16,7 +16,7 @@ class BaseSpatialOperations:
     spatial_version = None
 
     # How the geometry column should be selected.
-    select = None
+    select = '%s'
 
     @cached_property
     def select_extent(self):

django/contrib/gis/db/models/fields.py

@@ -266,8 +266,7 @@ class GeometryField(BaseSpatialField):
         of the spatial backend. For example, Oracle and MySQL require custom
         selection formats in order to retrieve geometries in OGC WKB.
         """
-        select = compiler.connection.ops.select
-        return select % sql if select else sql, params
+        return compiler.connection.ops.select % sql, params
 
 
 # The OpenGIS Geometry Type Fields