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