ui-call-data: Always use calls_best_match_get_avatar() to retrieve avatars

self->best_match is never NULL:
The test suite used to wrap calls_contacts_provder_new() to always return
NULL which in turn caused the best match to be NULL.

This was done to avoid warnings raised by libfolks
about missing the primary store (eds).

This is no longer necessary as the environment now tells folks which
backend to use.

src/calls-ui-call-data.c

@@ -145,10 +145,7 @@ calls_ui_call_data_get_avatar_icon (CuiCall *call_data)
 
   g_return_val_if_fail (CALLS_UI_CALL_DATA (self), NULL);
 
-  if (self->best_match)
-    return calls_best_match_get_avatar (self->best_match);
-  else
-    return NULL;
+  return calls_best_match_get_avatar (self->best_match);
 }