Avoid double destruction of wxTipWindow under wxOSX.

Avoid double destruction of wxTipWindow under wxOSX.

Hiding the window results in wxEVT_KILL_FOCUS under OS X and this destroys it
already, so when we call Destroy() from wxTipWindow::Close() later, it
triggers assert about destroying it twice.

Avoid doing this under OS X for now, even though the real problem is probably
the discrepancy in wxEVT_KILL_FOCUS event generation under different platforms.

Closes #14651.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72813 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775