Remove the hack with changing the parent background colour in wxInfoBar.

Remove the hack with changing the parent background colour in wxInfoBar.

Finding the closest info bar to copy the background colour from only worked
for very simple layouts and failed for more complicated ones with nested
sizers so don't try to change the colour automatically, it's easy enough to do
it manually as shown in the updated dialogs sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62301 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775