Remove one check from EnginioClientPrivate::networkManger().

The Enginio needs QNAM instance there is no point in delaying it
creation, especially with this:
https://codereview.qt-project.org/#change,56093 in mind.

We also do not need QPointer to protect QNAM instance access, the QNAM
is shared in a thread and it is not deleted as long as thread is alive.