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/db/src/bench.rs

@@ -507,7 +507,7 @@ impl<B: BlockT> StateBackend<HashFor<B>> for BenchmarkingState<B> {
 		*self.whitelist.borrow_mut() = new;
 	}
 
-	fn register_overlay_stats(&mut self, stats: &sp_state_machine::StateMachineStats) {
+	fn register_overlay_stats(&self, stats: &sp_state_machine::StateMachineStats) {
 		self.state.borrow_mut().as_mut().map(|s| s.register_overlay_stats(stats));
 	}