[kde-frameworks/kimageformats] Remove jpeg2k USE flag.

The jpeg2k plugin is now conditional on Qt <5.3 as later versions of Qt ship
their own plugin that is generally better.

Since we require Qt >=5.3 unconditionally we can just ignore this.

Upstream commit: 3d45b270ea8341d1516d5863cc49884c2744f2f2

Package-Manager: portage-2.2.12