Simplify `SyncingEngine::new()` a bit #907

Job	Run time
set-image	0s
check-runtime-assets	0s
check-runtime-bridge-hubs	0s
check-runtime-collectives	0s
check-runtime-coretime	0s
check-runtime-starters	0s
check-runtime-contracts	0s
check-runtime-testing	0s
All check-runtime-* tests passed	0s
	0s

Provide feedback