Remove unused _session setup; use the global session

Remove unused _session attribute from class initialization.

Author: zoghbi-a

astroquery/heasarc/core.py

@@ -38,7 +38,6 @@ def __init__(self):
         self._tap = None
         self._datalink = None
         self._meta_info = None
-        self._session = None
 
     @property
     def tap(self):