Consider the numpy array below. I'd hoping to find a fast way to remove rows not having 4 distinct values.

```
import numpy as np

D = np.array([[2, 3, 6, 7],
              [2, 4, 3, 4],
              [4, 9, 0, 1],
              [5, 5, 2, 5],
              [7, 5, 4, 8],
              [7, 5, 4, 7]])

```

In the small sample array show, the output should be:

```
D = np.array([[2, 3, 6, 7],
              [4, 9, 0, 1],
              [7, 5, 4, 8]])

```