Remove deleter for the kiva_backend property (which is readonly) (#1686)

PR #1670 added deleters for various ETSConfig attributes. However, the kiva_backend property is readonly, with value derived from the toolkit attribute, so we shouldn't be providing a deleter for it. The right way to mock kiva_backend is to set the toolkit attribute (and in fact there's already a test for that).

This PR removes the deleter for the kiva_backend attribute, and makes the `kiva_backend` property getter a pure function with no caching.

traits/etsconfig/etsconfig.py

@@ -321,25 +321,12 @@ def kiva_backend(self):
                 "which has not been set."
             )
 
-        if self._kiva_backend is None:
-            try:
-                self._kiva_backend = self._toolkit.split(".")[1]
-            except IndexError:
-                # Pick a reasonable default based on the toolkit
-                if self.toolkit == "wx":
-                    self._kiva_backend = (
-                        "quartz" if sys.platform == "darwin" else "image"
-                    )
-                elif self.toolkit in ["qt4", "qt"]:
-                    self._kiva_backend = "image"
-                else:
-                    self._kiva_backend = "image"
-
-        return self._kiva_backend
-
-    @kiva_backend.deleter
-    def kiva_backend(self):
-        self._kiva_backend = None
+        if "." in self._toolkit:
+            return self._toolkit.split(".")[1]
+        elif self.toolkit == "wx" and sys.platform == "darwin":
+            return "quartz"
+        else:
+            return "image"
 
     @property
     def user_data(self):

traits/etsconfig/tests/test_etsconfig.py

@@ -292,13 +292,13 @@ def test_toolkit_explicit_kiva_backend(self):
         self.ETSConfig.toolkit = "wx.celiagg"
         self.assertEqual(self.ETSConfig.kiva_backend, "celiagg")
 
-    def test_delete_kiva_backend(self):
-        # given
+    def test_toolkit_kiva_backend_changes_when_toolkit_changed(self):
         self.ETSConfig.toolkit = "wx.celiagg"
         self.assertEqual(self.ETSConfig.kiva_backend, "celiagg")
+        del self.ETSConfig.toolkit
 
-        # check that the property can be deleted
-        del self.ETSConfig.kiva_backend
+        self.ETSConfig.toolkit = "wx.quartz"
+        self.assertEqual(self.ETSConfig.kiva_backend, "quartz")
 
     def test_mock_kiva_backend(self):
         # when