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/regexp.go

@@ -112,22 +112,10 @@ var (
 	// are like digests without the algorithm, since sha256 is used.
 	IdentifierRegexp = regexp.MustCompile(identifier)
 
-	shortIdentifier = `([a-f0-9]{6,64})`
-	// ShortIdentifierRegexp is the format used to represent a prefix
-	// of an identifier. A prefix may be used to match a sha256 identifier
-	// within a list of trusted identifiers.
-	ShortIdentifierRegexp = regexp.MustCompile(shortIdentifier)
-
 	anchoredIdentifier = anchored(identifier)
 	// anchoredIdentifierRegexp is used to check or match an
 	// identifier value, anchored at start and end of string.
 	anchoredIdentifierRegexp = regexp.MustCompile(anchoredIdentifier)
-
-	anchoredShortIdentifier = anchored(shortIdentifier)
-	// anchoredShortIdentifierRegexp is used to check if a value
-	// is a possible identifier prefix, anchored at start and end
-	// of string.
-	anchoredShortIdentifierRegexp = regexp.MustCompile(anchoredShortIdentifier)
 )
 
 // literal compiles s into a literal regular expression, escaping any regexp