Amends 0c02d0545a that broke the
.qmlproject file: remove the unsupported prefer line from the qmldirs
and remove the now empty qmldir to make the .qmlproject file work. It
turns out that the .pro project does not require the prefer line either
from the qmldir files.
Pick-to: 6.8
Change-Id: I5202b589d7320542238a889db66c3975befceab6
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>