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

This reverts commit 4c0ac89a747f3ddb8d5acfe6696011c4b6007c6c.
Merge pull request #155 from amashigeseiji/t-1606-revert

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