[ImageView] Side histograms not clearing on main plot clear

A minimal working example to reproduce this issue

```python
import numpy as np
from silx.gui import qt
from silx.gui.plot.ImageView import ImageViewMainWindow


def main():
    app = qt.QApplication([])
    w = ImageViewMainWindow()
    w.addImage(np.random.random((512, 512)))
    w.clear()
    w.show()
    app.exec()


if __name__ == "__main__":
    main()
```