Remove unused torrent_pending_pieces.go

Unfortunately github.com/elliotchance/orderedmap is still used in ./analysis, so no deps are removed.

torrent_pending_pieces.go

@@ -1,40 +0,0 @@
-package torrent
-
-import (
-	"github.com/elliotchance/orderedmap"
-
-	"github.com/anacrolix/missinggo/v2/prioritybitmap"
-)
-
-type stableSet struct {
-	om *orderedmap.OrderedMap
-}
-
-func (s stableSet) Has(bit int) bool {
-	_, ok := s.om.Get(bit)
-	return ok
-}
-
-func (s stableSet) Delete(bit int) {
-	s.om.Delete(bit)
-}
-
-func (s stableSet) Len() int {
-	return s.om.Len()
-}
-
-func (s stableSet) Set(bit int) {
-	s.om.Set(bit, struct{}{})
-}
-
-func (s stableSet) Range(f func(int) bool) {
-	for e := s.om.Front(); e != nil; e = e.Next() {
-		if !f(e.Key.(int)) {
-			break
-		}
-	}
-}
-
-func priorityBitmapStableNewSet() prioritybitmap.Set {
-	return stableSet{om: orderedmap.NewOrderedMap()}
-}