Remove assert checking bitmap size in wxMenuItem drawing code in wxMSW.

Remove assert checking bitmap size in wxMenuItem drawing code in wxMSW.

This assert was fatal, as usual when asserting from a WM_PAINT handler, as the
function was reentered resulting in nested asserts and program abort, so
remove it to at least let the program continue to run even if there is not
enough space for the bitmap in the menu.

There is, of course, still something wrong with the menu geometry calculations
if this happens but I can't even reproduce this any more so not sure what
exactly.

See #11657.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71386 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775