fetch{url,Tarball}: Remove 'narHash' attribute

This was introduced in #6174. However fetch{url,Tarball} are legacy
and we shouldn't have an undocumented attribute that does the same
thing as one that already exists ('sha256').

src/libexpr/eval.hh

@@ -162,7 +162,7 @@ public:
     void allowPath(const StorePath & storePath);
 
     /* Allow access to a store path and return it as a string. */
-    void allowAndSetStorePathString(const StorePath & storePath, Value &v);
+    void allowAndSetStorePathString(const StorePath & storePath, Value & v);
 
     /* Check whether access to a path is allowed and throw an error if
        not. Otherwise return the canonicalised path. */