Windows: Don't disable JIT

[ChangeLog][QtQml][Windows] V4 JIT was re-enabled after it was disabled by mistake in 5.4.0.

Task-number: QTBUG-43171
Change-Id: I16206e094c210b1eadbfef9ee04cf78d3a7dcebe
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>