MAINT: Removes lookup_symbol_resolve_multiple method

lookup_symbol_resolve_multiple was identical to lookup_symbol, except that lookup_symbol performed upper-casing of the input string and lookup_symbol would return Nones. Now, lookup_symbol has a kwarg 'default_None=True' and all symbols are upper-cased on insertion and request.

zipline/algorithm.py

@@ -749,9 +749,10 @@ class TradingAlgorithm(object):
         _lookup_date = self._symbol_lookup_date if self._symbol_lookup_date is not None \
             else self.sim_params.period_end
 
-        return self.asset_finder.lookup_symbol_resolve_multiple(
+        return self.asset_finder.lookup_symbol(
             symbol_str,
-            as_of_date=_lookup_date
+            as_of_date=_lookup_date,
+            default_None=False,
         )
 
     @api_method