Migration 144: Fix drop index statement

When dropping an index related to a column, we need to make sure that
the `Index` object is removed the `Table` object, otherwise a subsequent
`drop_column` call to that same column will error out as it tries to
re-drop the index.

Change-Id: Ie6533b011ead1bc5dd2542041d50eb82e5654a55
Merge "Migration 144: Fix drop index statement"