Revert "fix imperfections of existence check of the model class in opPluginInstallTask::execute() (refs #3631, BP from #1606)"

This reverts commit 733605a956e735fb1d31ff92fdb42caff52d5253.
Merge pull request #154 from amashigeseiji/t-3631-revert

Revert "fix imperfections of existence check of the model class in opPluginInstallTask::execute() (refs #3631, BP from #1606)"