fix(difftool): add check for window number when cleaning up layout

[deathbeam]

• Move autocmd cleanup logic back to setup_layout as WinClosed autocmds were triggering for every closed window even when buf scoped, and when buf scoped, pattern filter did not work.
• Cleanup when quickfix window is closed as well to prevent state where on closing quickfix setup_layout will reset it after with empty content because of WinClosed autocmds triggering.

[justinmk]

It's usually a sign of a design issue when buffer-local autocmds aren't used. But I can't look at this closely to give specific advice.