Remove constraint addition from OpAlterColumn

[ryanslade]

Now that we have OpCreateConstraint we should remove the ability to add constraints via the OpAlterColumn type:

pgroll/pkg/migrations/types.go

Line 89 in 51b2641

type OpAlterColumn struct {

Having the ability to create constraints in two ways is confusing and the older method of using OpAlterColumn only supports single column constraints.

We should remove the following constraint fields:

• Check
• References
• Unique