Remove obsolete error messages in impex filters

All impex filters use now KisImportExportErrorCode
instead of BuilderResult or
KisImportExportFilter::ConversionStatus as it was
before. Since error codes mentioned above
were only used for impex filters, there is no need
to keep the code around now.

(cherry picked from commit e3e61bfa16e67f4bc1b0efa6c425cf8f3ad6222e)