Remove debuggerengine setting for Windows Phone

It's unclear whether a stock CDB can be used for Windows Phone in the
future (Qt Creator 3.1 does not support any debugging, currently).

Change-Id: Ib8e75a1796f1a4dac02c5ded8c776d6aac26be1b
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>