tests: Don't mock calls_contacts_provider_new()

Having set which folks backends to use the real function can be used
without causing any warnings leading to the build to fail.

tests/test-manager.c

@@ -5,18 +5,11 @@
  */
 
 #include "calls-manager.h"
-#include "mock-contacts-provider.h"
 
 #include <cui-call.h>
 #include <gtk/gtk.h>
 #include <libpeas/peas.h>
 
-CallsContactsProvider *
-__wrap_calls_contacts_provider_new (void)
-{
-  return NULL;
-}
-
 struct TestData {
   GMainLoop *loop;
   CallsManager *manager;