QTableView: prevent QTableView from hanging when removing rows.

QTableView: prevent QTableView from hanging when removing rows.

The problem was introduced in cd2afafb where we removed some code that
was meant to adjust the header's offset upon row removal.
The problem with this is that visualIndexAt() is likely to return -1 in
QHeaderView::paintEvent, which in turn will lead to calling paintSection
for each and every section.

Task-number: QTBUG-18551
Reviewed-by: Thierry
(cherry picked from commit d814e378987348ce2123d083b01ea6fb6c3e6bbf)