Revert "QString::lastIndexOf: fix off-by-one for zero length matches"
This reverts commit 3a273ac4.
See QTBUG-94215
6cee204d for qtbase/dev changes the behaviour even further, I'm pretty sure we don't want that in Qt 5.15.x, see discussion in https://codereview.qt-project.org/c/qt/qtbase/+/365179.
Change-Id: I663d74e0d44ebf46291fe0e8a7dc609be82eedc6