eventpluginsmanager: don't crash on double-delete of plugin objects

The Qt docs command no deletes!

> Don't try to delete the root component. Instead rely on that unload()
will automatically delete it when needed.

and

> When the library is finally unloaded, the root component will
automatically be deleted.

consequently don't delete the root component and then crash when a
double delete occurs. This notably seems easy to produce with multiple
clocks that have events plugins loaded.

BUG: 477165


(cherry picked from commit 0f1fe59a)