Previously, when creating a new instance of a sheet, the full set of references
was cleared.
Moreover, if this sheet has sub-sheets, the annotation was incorrectly handled
Now only (and all) new sheet path(s) created have a reference cleared, as expected.
(new sheet paths can be more than one, if the new instance of the sheet has sub-sheets)
Fixes: lp:1789048
https://bugs.launchpad.net/kicad/+bug/1789048