This step is optional. The deletion of the installation package is fully defined by the uninstall scripts of the 'wrapper' RPM (or DEB, or SH) package. A special uninstall script is created and added to the /uninstall folder of the installation package in order to solve some non-trivial tasks. E.g., the RPM/SH/DEB package just deletes files and folders of the installation package from the system, while the uninstall script can be used for cleaning out some other traces of the application's presence in the system.
When Plesk receives a command to delete an installation package from AV Repository, it searches for the uninstall script in the /uninstall folder of the installation package and executes it. If the uninstall script is not found, the uninstall procedure will not fail, but continue as defined by the RPM/SH/DEB package.
Thus, this step should result in the uninstall script added to the /uninstall folder of the installation package if any specific 'delete' actions are necessary.