Don't delete child controls when deleting wxStaticBoxSizer.

This is an incompatible change compared to 2.8 which can make the existing
code crash and it also goes against the usual rule that the windows are never
owned by sizers, only other windows.

Closes #15698.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75922 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775