modules/markup/markdown/ast: Drop IsColorPreview

The function isn't used anywhere, it's dead code. Remove it.

Partially cherry picked from commit 46c3f3b9e81e25c3ed08ba6c5bfeb8802b3ba1fd.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

modules/markup/markdown/ast.go

@@ -174,10 +174,3 @@ func NewColorPreview(color []byte) *ColorPreview {
 		Color:      color,
 	}
 }
-
-// IsColorPreview returns true if the given node implements the ColorPreview interface,
-// otherwise false.
-func IsColorPreview(node ast.Node) bool {
-	_, ok := node.(*ColorPreview)
-	return ok
-}