This removes the code unneeded in wxUSE_THREADS==0 build and also fixes
compilation of wxGUIAppTraits::WaitForThread() which didn't compile any more
with wxUSE_THREADS==0 since r67185.
Closes#13050.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67288 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775