MAINT: Remove unused member.

The "in before trading start logic" is now all handled inside of
`BarData`.

zipline/data/data_portal.py

@@ -528,8 +528,6 @@ class DataPortal(object):
         }
         self._equity_minute_loader_array_data = {}
 
-        self._in_bts = False
-
     def _reindex_extra_source(self, df, source_date_index):
         return df.reindex(index=source_date_index, method='ffill')