fdo#71628 Startcenter: remove thumbnail selection when focus is moved away

mnHighItemId is never assaigned with a valid value and there is
no sense to store one selected element's index anyway,
because there can be more.
The bMove parameter of ImplGetItem is never used and doesn' work
so remove it.

Change-Id: I91d6bc3fd13a2b44e6961fea66d02702a8f31c2b
(cherry picked from commit 0314034d8b23d5b69399cfcdf6b2f2add73f9f47)
Reviewed-on: https://gerrit.libreoffice.org/7340
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>