bugfix: orphans are not totally collected when Remove

[rickyyangz]

Details:

Summary:

Remove operation will leave some orphans not collected. So these orphans would never be pruned from database. So many nodeDB operations would get the wrong result, like traverse, traversePrefix, traverseNodes, size(), etc.

Steps To Reproduce:

1. Run the test code

func TestMutableTree_Remove(t *testing.T) {
	db := db.NewDB("test", db.MemDBBackend, "")
	tree := NewMutableTree(db, 0)
	tree.Set([]byte("k1"), []byte("v1"))
	tree.Set([]byte("k2"), []byte("v1"))
	tree.Set([]byte("k3"), []byte("v1"))
	tree.Set([]byte("k4"), []byte("v1"))
	tree.Set([]byte("k5"), []byte("v1"))
	tree.SaveVersion()
	tree.Remove([]byte("k5"))
	require.Equal(t, 4, len(tree.orphans))
	tree.SaveVersion()
}

The test will failed because the Remove only collect 3 orphans instead of the expected 4.
The tree looks like this

           3                                                3
         /   \                                            /   \    
        2      4                                         2     4
      /  \    /  \                =>                   /  \   /  \
     1    2  3    5                                  1   2  3    4
                 /  \
                4    5

The orphans should be [5, 5, 4, 3], but we got [5,4,3] instead. The inner node 5 is lost.

[AdityaSripal]

Thanks for this fix!