CAN: merge the two Q_GLOBAL_STATICs for plugins

We don't need two independent variables, each with their own creation
guard. One suffices. Fold them into a single struct whose constructor
builds the map from the loader's metadata, so the two always come and go
together and the free function disappears.

This also removes the stored QCborMap containing the plugins'
metadata. They were never consumed.

Pick-to: 6.12
Change-Id: I86455bc436c695343986fffd249a7b68d423c287
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>