[R-package] [ci] remove 'Date' field from DESCRIPTION #7396

Contributes to #6809

Contributes to #6774

While reviewing #7392, I realized ... we probably don't need the Date: field in R-package/DESCRIPTION in source control, and could remove the machinery for updating it at build time 😁

It looks like Date is not required and has not been for a long time. Look at this comment from 2016: https://github.com/r-lib/devtools/issues/1327#issuecomment-244590948

That exactly matches what I see looking around at popular R packages. For example, {arrow} does not have a Date: field in its description (apache/arrow - r/DESCRIPTION) but a Date/Publication date is added in the CRAN package (code link).

Notes for Reviewers

If R CMD check --as-cran runs successfully in CI here, I think we can be confident in this change.