cue/errors: remove Augment

No longer needed.

Change-Id: Icbce645970087cd24750cfdda181ed8543c7b042
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2204
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/cue/errors/errors.go b/cue/errors/errors.go
index 99198e0..1531f51 100644
--- a/cue/errors/errors.go
+++ b/cue/errors/errors.go
@@ -115,16 +115,6 @@
 	return e
 }
 
-// Augment adorns an existing error with new information.
-func Augment(err error, args ...interface{}) error {
-	e, ok := err.(*posError)
-	if !ok {
-		e = &posError{err: err}
-	}
-	update(e, args)
-	return e
-}
-
 func update(e *posError, args []interface{}) {
 	err := e.err
 	for _, a := range args {