lint: Update database.g.dart to suppress unnecessary_cast too

This happens automatically if you rerun build_runner.  Seems like
its omission was basically a mismerge between 901a99e, when it
was merged early from zulip#84:
  zulip#84 (comment)
and 780b092 / zulip#22, which added this file.

Chalk it up as another occasion where zulip#60 would have helped keep
things clean.