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/primitives/state-machine/src/ext.rs

@@ -682,7 +682,7 @@ where
 			self.overlay.rollback_transaction().expect(BENCHMARKING_FN);
 		}
 		self.overlay.drain_storage_changes(
-			&self.backend,
+			self.backend,
 			#[cfg(feature = "std")]
 			None,
 			Default::default(),
@@ -700,7 +700,7 @@ where
 			self.overlay.commit_transaction().expect(BENCHMARKING_FN);
 		}
 		let changes = self.overlay.drain_storage_changes(
-			&self.backend,
+			self.backend,
 			#[cfg(feature = "std")]
 			None,
 			Default::default(),