reference: remove support for deprecated "shortid" refs

The "shortid" syntax was added in d26a3b37a6,
and allowed for matching an image on its ID prefix (this is before images were
content-addressable). With the introduction of content-addressable references,
this syntax became problematic, and Docker deprecated this syntax in 2016
(Docker v1.13.0) through commit; 5fc71599a0

> The `repository:shortid` syntax for referencing images is very little used,
> collides with tag references, and can be confused with digest references.

Support for this syntax was removed in 2017 (Docker 17.12) through commit:
a942c92dd7

containerd uses a fork of the reference package with this syntax removed, and
does not support this syntax:
901bcb2231

This patch removes the deprecated syntax, the ParseAnyReferenceWithSet function,
and the ShortIdentifierRegexp regex.

As there are no external consumers for this function, nor the regexp, I'm
skipping a deprecation cycle for this;

- https://grep.app/search?q=.ShortIdentifierRegexp
- https://grep.app/search?q=.ParseAnyReferenceWithSet%28

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

reference/normalize.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/distribution/distribution/v3/digestset"
 	"github.com/opencontainers/go-digest"
 )
 
@@ -179,20 +178,3 @@ func ParseAnyReference(ref string) (Reference, error) {
 
 	return ParseNormalizedNamed(ref)
 }
-
-// ParseAnyReferenceWithSet parses a reference string as a possible short
-// identifier to be matched in a digest set, a full digest, or familiar name.
-func ParseAnyReferenceWithSet(ref string, ds *digestset.Set) (Reference, error) {
-	if ok := anchoredShortIdentifierRegexp.MatchString(ref); ok {
-		dgst, err := ds.Lookup(ref)
-		if err == nil {
-			return digestReference(dgst), nil
-		}
-	} else {
-		if dgst, err := digest.Parse(ref); err == nil {
-			return digestReference(dgst), nil
-		}
-	}
-
-	return ParseNormalizedNamed(ref)
-}