MAINT: Remove empty inputs from BoundColumn.

They belong on LoadableTerm instead.

zipline/pipeline/data/dataset.py

@@ -112,7 +112,6 @@ class BoundColumn(LoadableTerm):
         The name of this column.
     """
     mask = AssetExists()
-    inputs = ()
     window_safe = True
 
     def __new__(cls, dtype, missing_value, dataset, name):

zipline/pipeline/term.py

@@ -345,6 +345,7 @@ class LoadableTerm(Term):
     This is the base class for :class:`zipline.pipeline.data.BoundColumn`.
     """
     windowed = False
+    inputs = ()
 
     @lazyval
     def dependencies(self):