Remove now-unnecessary download of nlohmann_json header #3
Description
A couple of days ago, Pengfei installed nlohmann_json as a ups product, which meant that it was possible to remove a hack in appfwk's CMake code where the nlohmann_json header was directly downloaded. I added this hack to the CMake logic in the cogs build the week before last since nlohmann_json wasn't available as a ups product at the time (Issue #1); now, analogous to what I did with appfwk, I should remove it from cogs.