Remove constraints before removing column.

[Soulforged]

The adapter was not making sure that there were no constraints on a column before attempting to remove it.

[mjaric]

Hi @Soulforged,

Thank you for your contribution. Unfortunately I don't think this should be implicit action, migration should be explicit in things they do and represent developer intention especially if you want them to drop things. Also note that ecto's default adapters, mysql and postgres, do not drop implicitly constraints so I would rather keep same behaviour