Don't show download size for Flatpak apps

- Most of the time the numbers are similar, or at least not different
  enough to meaningfully matter
- It takes up a lot of room on the header, and looks awkward
- It's inconsistent with the PackageKit backend, which doesn't do this
  even though the download size also differs from the installed size

BUG: 475849
FIXED-IN: 6.0