Remove Column field from DropConstraint

[ryanslade]

We can look this up from the schema so can remove it from the spec.

This is a breaking change since we remove a field from the schema.

Part of #105

[andrew-farries]

The changes here are fine.

JSON validation rejects unknown fields so existing drop_constraint operations that include the column field will no longer be valid.

This will require existing migrations to be edited otherwise they won't apply.

[ryanslade]

Do you mean existing migrations that exist within xata?

[andrew-farries]

Do you mean existing migrations that exist within xata?

Yes, any existing branches that have a drop constraint migration in their history will fail to have their migations replayed when a user creates a new branch.