Remove wrong warning about non-rgb resource images

It is perfectly fine, in fact, by design that some resources (notably
brushes) are QImage::Format_Indexed, so we shouldn't warn if the user
selects one of those in the brush editor.

I have reviewed all places where KoResource::image() was used and found
that the only place where there was code expecting an RGB image was
KoResourceItemChooser::updatePreview, and that is in when we convert
an rgb image to grayscale in case the d->grayscalePreview is true.

That is only the case in the brush editor's texture selector.

This also means we can handle non-rgb patterns again -- many are mono
or indexed, and that is fine.