decoder: refactor SelfRefs use context

[giautm]

This PR refactors a mess in the codebase with allowSelfRefs passing around until it reaches the place it was used for.

The helper schema.WithActiveSelfRefs(ctx) is already defined by the author, so I just use it for this suppose; and remove the allowSelfRefs variable from the method.

Before:

type ReferenceOriginsExpression interface {
	ReferenceOrigins(ctx context.Context, allowSelfRefs bool) reference.Origins
}

After:

type ReferenceOriginsExpression interface {
	ReferenceOrigins(ctx context.Context) reference.Origins
}

[giautm]

@dbanck: please help to review

[jpogran]

Hi @giautm, thank you for offering a contribution to this repo. Can you please put a description of your change in the PR, along with the reasoning for your change? Right now the PR description is empty and the commit message just points to giautm#2, which I did not find helpful either.

In general it is a good practice to open an issue describing the change you want so there is a chance for us to discuss the design before work is put into a PR that may not be accepted as is. Apologies, this repo does not have a CONTRIBUTING.md to have set that expectation, I will fix that.

Once we have an understanding of the intent of the change, we can review this PR.

[giautm]

@jpogran updated its description, thank you.