Delete cef_web_plugin.h and plugin-related APIs (see issue #3047)

This functionality stopped being relevant after the removal of Flash support
in January 2021. The last remaining PPAPI plugin (PDF viewer) will switch to
a non-plugin implementation (PdfUnseasoned) in M100.

include/cef_context_menu_handler.h

@@ -277,7 +277,7 @@ class CefContextMenuParams : public virtual CefBaseRefCounted {
 
   ///
   // Returns true if the context menu contains items specified by the renderer
-  // process (for example, plugin placeholder or pepper plugin menu items).
+  // process.
   ///
   /*--cef()--*/
   virtual bool IsCustomMenu() = 0;