Update AIT-GUI for AIT-Core 3 / msgpack serialization

[nttoole]

As part of NASA-AMMOS/AIT-Core#542, pickle was replaced by msgpack for serialization.

During the pickle era, some messages were double-pickled (pickled prior to passing off the 0MQ where it would be pickled again). Plugin's would eventually receive a pickled-once message, and so would need to call pickle.loads() to recover.
With AIT-Core 3, the message passed to Plugin.process() is fully recovered. So we just need to ensure pickle.loads() is removed.

AIT-GUI/ait/gui/__init__.py

Line 352 in ac75c76

def process_telem_msg(self, msg):