downloadFile(): Remove the "locked" (aka "immutable") flag

This was used in only one place, namely builtins.fetchurl with an
expected hash. Since this can cause similar issues as described
in #9814 and #9905 with the "locked" flag for fetchTarball and fetchTree,
let's just remove it.

Note that if an expected hash is given and the hash algorithm is
SHA-256, then we will never do a download anyway if the resulting
store path already exists. So removing the "locked" flag will only
cause potentially unnecessary HTTP requests (subject to the tarball
TTL) for non-SHA-256 hashes.

src/libexpr/primops/fetchTree.cc

@@ -473,7 +473,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     auto storePath =
         unpack
         ? fetchToStore(*state.store, fetchers::downloadTarball(*url).accessor, FetchMode::Copy, name)
-        : fetchers::downloadFile(state.store, *url, name, (bool) expectedHash).storePath;
+        : fetchers::downloadFile(state.store, *url, name).storePath;
 
     if (expectedHash) {
         auto hash = unpack