Remove QJSValue::toInteger() function

Remove QJSValue::toInteger() function

Rationale: This is a remnant from QtScript. A function called
toInteger() that returns a double looks strange.
Use toInt32() to convert a QJSValue to an integer.

Task-number: QTBUG-23604

Change-Id: I2829704c64b077fca264b660c46248c3f35cb5c0
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>