gateway_base: remove unused ExecModel _count attribute

Author: bluetech

execnet/gateway_base.py

@@ -92,7 +92,6 @@ class ExecModel:
         def __init__(self, name):
             self._importdef = importdef
             self.backend = name
-            self._count = 0
 
         def __repr__(self):
             return "<ExecModel %r>" % self.backend