[pkcon] Remove '-y' assignment from "only-download"

The '-y' option is already used for "noninteractive", so we cannot
use it for "only-download" as well (otherwise it would not be possible
to have a non-interactive installation of packages).

Upstream bug: https://bugs.freedesktop.org/show_bug.cgi?id=70025
Merge pull request #11 from thp/pkcon-y-option

[pkcon] Remove '-y' assignment from "only-download"