[errors] Greatly simplify common/errors package.

common/errors/walk.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package errors
 
 // Walk performs a depth-first traversal of the supplied error, unfolding it and
 // invoke the supplied callback for each layered error recursively. If the
 // callback returns true, Walk will continue its traversal.
 //
@@ skipping 37 matching lines @@
 // visited erorr.
 //
 // If err is nil, Any will return false.
 func Any(err error, fn func(error) bool) (any bool) {
         Walk(err, func(err error) bool {
                 any = fn(err)
                 return !any
         })
         return
 }
-
-// Contains performs a Walk traversal of an error, returning true if it is or
-// contains the supplied sentinel error.
-func Contains(err, sentinel error) bool {
-        return Any(err, func(err error) bool {
-                return err == sentinel
-        })
-}