ThumbnailView: item found by mouse pos is always visible

-- so it does not need to check it again

Change-Id: Id6fc2732bde8dd7f6011828b6bc29d7947bf6312
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/6629
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>