Remove 7dav cumulative signals from JHU documentation and from the DB

[nmdefries]

In JHU Cases and Deaths, the documentation still lists the silly signals confirmed_7dav_cumulative_num and confirmed_7dav_cumulative_prop -- it doesn’t make much sense to average cumulative data. We should remove these from the documentation, and also from the DB. Of note, the parallel source USAFacts does not have these (compare the two signal lists). I [Roni] have a vague recollection that I pointed this out during the pandemic, and it was removed, but maybe only from USAFacts and not from JHU.