Make light client backend only work with locally available data (#3538)

* removing fetcher dependency from light backend

* fix compilation

substrate/core/test-client/src/lib.rs

@@ -47,7 +47,6 @@ use client::LocalCallExecutor;
 /// Test client light database backend.
 pub type LightBackend<Block> = client::light::backend::Backend<
 	client_db::light::LightStorage<Block>,
-	LightFetcher,
 	Blake2Hasher,
 >;