drop column is sometimes broken

[abe-winter]

example failure:

psycopg2.errors.SyntaxError: syntax error at or near "bool"
LINE 6: alter table users drop column notify_email bool;

obvious why this is invalid sql and what the change generator is doing, but I think I've also seen drops emit correct sql.

• make sure this is covered in a test case
• see if the AST is being used correctly by the diff generator

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also test rm enum (both for drop order and syntax error)