test: remove CollectionHandler from test helpers

Remove the CollectionHandler field from router.Config struct literal
in cmd/server/tests/test_helpers.go. This field was removed from
the Config struct in a previous commit.

The collection routes are registered directly in handlers.SetupRoutes()
and don't need to be passed through the router config.

cmd/server/tests/test_helpers.go

@@ -126,7 +126,6 @@ func setupTestServer(t *testing.T) (*httptest.Server, *database.Queries, *config
 		ConflictHandler:      conflictHandler,
 		AnalyticsHandler:     analyticsHandler,
 		QueueHandler:         queueHandler,
-		CollectionHandler:    nil, // Not needed for tests
 		OPDSHandler:          opdsHandler,
 		ConnManager:          connManager,
 		QueueProcessor:       queueProcessor,