Fix MAPDL collection

[akaszynski]

Resolve #800 by removing the additional reference to the mapdl instance in ansys/mapdl/core/logging.py.

@germa89, note that adding self.extra = extra created an additional reference to our mapdl instance, which then created a circular reference since our mapdl instance contained a reference to the logger, which contained a reference to the mapdl instance.

Once this is merged, let's plan to push a release.

[germa89]

So weakref is the way to go in those cases.