Unsupported psd layers were dropped when loading psd images. This
causes problems when handling psd clipping groups, see issue #8453.

PSP also does not drop unsupported layers, so let's keep the layers
and accept that they will usually show up empty. This will help in
fixing clipping groups in a next step.

While we are at it, also remove the warnings that are only shown when
define CONVERSION_WARNINGS is TRUE (defaults to FALSE). We already
have a better way to show unsupported features, so at least for the
unsupported layers it is not needed anymore.
There are still some CONVERSION_WARNINGS warnings left over in the
blend modes conversion, which do not have a similar warning in our
unsupported features dialog.