Target: Avoid needless switching of active RunConfigurations.

Together with the previous patch this avoids recreating RunConfiguration
widgets for RCs which are about to be removed because they are no longer
valid.

Task-number: QTCREATORBUG-10248
Change-Id: Ia8e394175c1cdef469faa8d117a2736500d928e4
Reviewed-by: Daniel Teske <daniel.teske@digia.com>