fix(replica): best-effort to delete old head disk file

The deletion of old head disk file is a best-effort task, and
the failure should not hinder the following tasks.

Longhorn 9168

Signed-off-by: Derek Su <derek.su@suse.com>
(cherry picked from commit 19d3962)

pkg/replica/replica.go

@@ -920,8 +920,9 @@ func (r *Replica) createDisk(name string, userCreated bool, created string, labe
 	rollbackFuncList := []func() error{}
 	defer func() {
 		if err == nil {
-			err = r.rmDisk(oldHead)
-			logrus.WithError(err).Errorf("Failed to remove old head %v", oldHead)
+			if errRm := r.rmDisk(oldHead); errRm != nil {
+				logrus.WithError(errRm).Warnf("Failed to remove old head %v", oldHead)
+			}
 			return
 		}