[storage] Removed unused references

Unused references have become a build warning in the rust compiler and
as warning are errors in our build, this is blocking the rust toolchain
from rolling.
See rust-lang/rust#76894

Change-Id: Ifcbdcc93efa4ca53003c99515767b58dc3675194
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/545184
Commit-Queue: Adrian Danis <adanis@google.com>
Fuchsia-Auto-Submit: Adrian Danis <adanis@google.com>
Reviewed-by: Chris Suter <csuter@google.com>

Author: AdrianDanis

src/lib/storage/block_client/rust/src/testing.rs

@@ -123,7 +123,7 @@ impl BlockClient for FakeBlockClient {
             BufferSlice::Memory(slice) => {
                 let len = slice.len();
                 ensure!(device_offset + len <= inner.data.len(), "Invalid range");
-                &mut inner.data[device_offset..device_offset + len].copy_from_slice(slice);
+                inner.data[device_offset..device_offset + len].copy_from_slice(slice);
                 Ok(())
             }
         }