Remove recommendation for avoiding -j when building CRETE

[moralismercatus]

Previously, there was a dependency issue (or some other issue) with the CRETE build that disallowed the use of -jN. Since the recent changes to the CMake files, I have been able to use -jN without issue (greatly reducing build times).

I'd like further confirmation on this. Once decided that the issue is no longer present, we can remove the recommendation against -j in the documents.

[moralismercatus]

@likebreath Seems you can close this with the latest rebase.