Removes unnecessary blank impl for Backend (#8897)

* Removes unnecessary blank impl for Backend

This commit removes a from my perspective unneccessary implementation
for &T which implement Backend.

The current implementation exists (again from my perspective) solely
to satisfy a methods &mut self parameters (i.e. allows to satisfy
this for an & reference via using &mut &Backend).

As all implementors use a RefCell with borrow_mut() where actually
calling the mentioned &mut self method and then forwad to the
{} implementation of either TrieBackend or ProvingBackend, the
current &mut self seems to be not needed.

* Fixed tests client

substrate/client/service/test/src/client/mod.rs

@@ -167,7 +167,7 @@ fn construct_block(
 	};
 	let hash = header.hash();
 	let mut overlay = OverlayedChanges::default();
-	let backend_runtime_code = sp_state_machine::backend::BackendRuntimeCode::new(&backend);
+	let backend_runtime_code = sp_state_machine::backend::BackendRuntimeCode::new(backend);
 	let runtime_code = backend_runtime_code.runtime_code().expect("Code is part of the backend");
 	let task_executor = Box::new(TaskExecutor::new());