replace usages of backend.Remove() with repository.RemoveUnpacked()

RemoveUnpacked will eventually block removal of all filetypes other than
snapshots. However, getting there requires a major refactor to provide
some components with privileged access.

internal/repository/repack_test.go

@@ -167,7 +167,7 @@ func repack(t *testing.T, repo restic.Repository, packs restic.IDSet, blobs rest
 	}
 
 	for id := range repackedBlobs {
-		err = repo.Backend().Remove(context.TODO(), backend.Handle{Type: restic.PackFile, Name: id.String()})
+		err = repo.RemoveUnpacked(context.TODO(), restic.PackFile, id)
 		if err != nil {
 			t.Fatal(err)
 		}

internal/repository/repository.go

@@ -520,6 +520,11 @@ func (r *Repository) verifyUnpacked(buf []byte, t restic.FileType, expected []by
 	return nil
 }
 
+func (r *Repository) RemoveUnpacked(ctx context.Context, t restic.FileType, id restic.ID) error {
+	// TODO prevent everything except removing snapshots for non-repository code
+	return r.be.Remove(ctx, backend.Handle{Type: t, Name: id.String()})
+}
+
 // Flush saves all remaining packs and the index
 func (r *Repository) Flush(ctx context.Context) error {
 	if err := r.flushPacks(ctx); err != nil {