Remove calls to non-existent figure methods in MPL code (#1382)

* Remove call to `fig.strip_style` (was removed in v3)
* Remove call to `fig.validate` (was removed in v3)

Author: jonmmease

Diff for: plotly/matplotlylib/renderer.py

@@ -735,5 +735,4 @@ def resize(self):
                 pass
 
     def strip_style(self):
-        self.msg += "Stripping mpl style, deleting keys from data and layout\n"
-        self.plotly_fig.strip_style()
+        self.msg += "Stripping mpl style is no longer supported\n"

Diff for: plotly/plotly/plotly.py

@@ -276,7 +276,6 @@ def iplot_mpl(fig, resize=True, strip_style=False, update=None,
     fig = tools.mpl_to_plotly(fig, resize=resize, strip_style=strip_style)
     if update and isinstance(update, dict):
         fig.update(update)
-        fig.validate()
     elif update is not None:
         raise exceptions.PlotlyGraphObjectError(
             "'update' must be dictionary-like and a valid plotly Figure "