Remove duplicated _secondary_min method

pygal/graph/base.py

@@ -184,13 +184,6 @@ class BaseGraph(object):
         return (self.range and self.range[0]) or (
             min(self._values) if self._values else None)
 
-    @cached_property
-    def _secondary_min(self):
-        """Getter for the secondary minimum series value"""
-        return (self.range and self.range[0]) or (
-            min(self._secondary_values) if self._secondary_values else None)
-
-
     @cached_property
     def _max(self):
         """Getter for the maximum series value"""