Don't reset previous window event handler in PopEventHandler().

Don't reset previous window event handler in PopEventHandler().

This is harmless but also useless as the previous event handler of a wxWindow
is always NULL anyhow, so simply don't do this. This is consistent with
PushEventHandler() which doesn't call SetPreviousHandler() for the window
itself neither.

Closes #14353.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71621 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775