Deprecate not working wxSplitterWindow::SetSashSize().

Setting sash size to non default value didn't work correctly and didn't make
much sense anyhow as the sash appearance is platform-dependent and current
code for drawing it doesn't work for arbitrary sizes.

Simply remove the possibility to set the sash size.

Closes #12412.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66232 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/splitter.h

@@ -136,6 +136,18 @@ public:
     */
     int GetSashPosition() const;
 
+    /**
+        Returns the sash size in pixels.
+
+        The size of the sash is its width for a vertically split window and its
+        height for a horizontally split one. Its other direction is the same as
+        the client size of the window in the corresponding direction.
+
+        The sash size is platform-dependent because it conforms to the current
+        platform look-and-feel and cannot be changed.
+     */
+    int GetSashSize() const;
+
     /**
         Gets the split mode.
 
@@ -285,15 +297,6 @@ public:
     */
     void SetSashPosition(int position, bool redraw = true);
 
-    /**
-        Sets the sash size. Normally, the sash size is determined according to the
-        metrics
-        of each platform, but the application can override this, for example to show
-        a thin sash that the user is not expected to drag. If @a size is more -1,
-        the custom sash size will be used.
-    */
-    void SetSashSize(int size);
-
     /**
         Sets the split mode.