Flickable containing MultiPointTouchArea is unresponsive following touch.

Flickable containing MultiPointTouchArea is unresponsive following touch.

MPTA was releasing the mouse grab when all touches were
released, causing the mouse release event to not be propagated
to its parents, who were then unable to correct their state.
Remove this code and allow the normal event handlers to do their
thing.

This is the same fix as was implemented for PinchArea in
9634dc440269fc03f825a6d18b371d8e612ba9ec

Change-Id: I2715677157f5838e3f81137f64765024cf2f0459
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>