cp: remove unnecessary unwrap

Had assumed our MSRV did not support let-chains; happy to stand corrected.

Author: Alonely0

src/uu/cp/src/copydir.rs

@@ -204,10 +204,8 @@ impl Entry {
                         translate!("cp-error-failed-to-create-directory", "error" => e)
                     );
                 }
-            } else if let Ok(stripped) =
-                // The following unwrap is unreachable because context.root is always *something*.
-                descendant
-                    .strip_prefix(context.root.components().next_back().unwrap())
+            } else if let Some(prefix) = context.root.components().next_back()
+                && let Ok(stripped) = descendant.strip_prefix(prefix)
             {
                 descendant = stripped.to_path_buf();
             }