Delete a superfluous (deprecated) JSONReader call

I left this part out from the previous crrev.com/c/2222055, which added
new code but forgot to delete the deprecated code.

Bug: 1070409
Change-Id: Id84bab3ac58b8e0bab979a4fb8ffacbd5d7faa68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227941Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774550}

components/sync_preferences/pref_model_associator.cc

@@ -445,9 +445,6 @@ base::Optional<base::Value> PrefModelAssociator::ReadPreferenceSpecifics(
     const sync_pb::PreferenceSpecifics& preference) {
   base::JSONReader::ValueWithError parsed_json =
       base::JSONReader::ReadAndReturnValueWithError(preference.value());
-  base::JSONReader reader;
-  std::unique_ptr<base::Value> value(
-      reader.ReadToValueDeprecated(preference.value()));
   if (!parsed_json.value) {
     std::string err =
         "Failed to deserialize preference value: " + parsed_json.error_message;