Remove asserts in wxChoicebook::Get/SetPageImage().

Remove asserts in wxChoicebook::Get/SetPageImage().

It's better to silently return an error code than assert when these methods
are called as their failure is relatively harmless and the user code may call
them via base class pointer without caring if it works with a control which
supports images or not.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62895 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775