Minor code cleanup (#3901)

Remove now-unused code

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #2360

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/wrapper.go

@@ -129,11 +129,6 @@ func (w *Wrapper) Close(ctx context.Context) error {
 	return nil
 }
 
-type IncrementalBase struct {
-	*snapshot.Manifest
-	SubtreePaths []*path.Builder
-}
-
 // ConsumeBackupCollections takes a set of collections and creates a kopia snapshot
 // with the data that they contain. previousSnapshots is used for incremental
 // backups and should represent the base snapshot from which metadata is sourced