Remove "in 2020" wording from NCHS mortality signal description

[nmdefries]

In NCHS Mortality, the “deaths as percent of” signal is described as “Number of weekly new deaths for all causes in 2020 compared to the average number across the same week in 2017–2019” (boldfacing mine). I don’t think “in 2020” should be there, because this signal is ongoing since 2020.