Remove check for axes.hold

It has been deprecated in matplotlib for a long time
and is removed in 3.0. Callers should clear the axes
themselves if thay is appropriate.

Framework/PythonInterface/mantid/plots/modest_image/modest_image.py

@@ -211,8 +211,6 @@ def imshow(axes, X, cmap=None, norm=None, aspect=None,
 
     Unlike matplotlib version, must explicitly specify axes
     """
-    if not axes._hold:
-        axes.cla()
     if norm is not None:
         assert(isinstance(norm, mcolors.Normalize))
     if aspect is None: