Column dropping operation should use attnum

[mathemancer]

Problem

Currently, the column dropping function uses the column index to identify columns. This is brittle, and we've decided to change to using the attnum property of columns instead.

Proposed solution

The function in db/columns/operations/drop.py should use attnum instead of column_index, and should properly use the table_oid, attnum pair to identify columns for its logic. Use the function get_column_name_from_attnum in the db.columns.operations.select module for this.

Additional context

We'll need to modify any place where the function is called as well.