Remove all mentions of wxGauge shadow width and bezel face.

The shadow width was only used by wxMotif and bezel face not used at all since
a very, very long time, so just remove these methods from the ports which still
had them (just doing nothing) and remove support of the corresponding XRC
attributes.

docs/doxygen/overviews/xrc_format.h

@@ -1045,9 +1045,9 @@ objects. If sizer child is used, it sets
 @row3col{value, integer,
     Initial value of the control (default: 0).}
 @row3col{shadow, @ref overview_xrcformat_type_dimension,
-    Rendered shadow size (default: none; ignored by most platforms).}
+    Ignored, preserved only for compatibility.}
 @row3col{bezel, @ref overview_xrcformat_type_dimension,
-    Rendered bezel size (default: none; ignored by most platforms).}
+    Ignored, preserved only for compatibility.}
 @endTable
 
 @subsubsection xrc_wxgenericdirctrl wxGenericDirCtrl