Setting sash size to non default value didn't work correctly and didn't make
much sense anyhow as the sash appearance is platform-dependent and current
code for drawing it doesn't work for arbitrary sizes.
Simply remove the possibility to set the sash size.
Closes#12412.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66232 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775