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_test.go

@@ -550,43 +550,7 @@ func TestIdentifierRegexp(t *testing.T) {
 			match: false,
 		},
 	}
-
-	shortCases := []regexpMatch{
-		{
-			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf9821",
-			match: true,
-		},
-		{
-			input: "7EC43B381E5AEFE6E04EFB0B3F0693FF2A4A50652D64AEC573905F2DB5889A1C",
-			match: false,
-		},
-		{
-			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf",
-			match: true,
-		},
-		{
-			input: "sha256:da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf9821",
-			match: false,
-		},
-		{
-			input: "da304e823d8ca2b9d863a3c897baeb852ba21ea9a9f1414736394ae7fcaf98218482",
-			match: false,
-		},
-		{
-			input: "da304",
-			match: false,
-		},
-		{
-			input: "da304e",
-			match: true,
-		},
-	}
-
 	for i := range fullCases {
 		checkRegexp(t, anchoredIdentifierRegexp, fullCases[i])
 	}
-
-	for i := range shortCases {
-		checkRegexp(t, anchoredShortIdentifierRegexp, shortCases[i])
-	}
 }