Unless those projects have required copyright assignments (neither does, to my knowledge), then you’d need to get agreement from the copyright holder for every contribution that still has code in the project. If there are any holdouts (or even any people you just couldn’t get in touch with) then you’d need to remove the parts they contributed to proceed.
More practically, anyone who was asked could reasonably respond “Why do you need to own the copyright? It’s already permissively licensed (in jemalloc’s case, with an LGPLv2.1-compatible license) - just use it!” FSF policy is the only obstacle, and they could make an exception if they wanted to.
