refactor(win_close): remove "force", don't pass on "free_buf" (#21921)

Problem:
The "force" flag of win_close() complicates the code and adds edge cases
where it is not clear what the correct behavior should be.
The "free_buf" flag of win_close() is passed on to float windows when
closing the last window of a tabpage, which doesn't make much sense.

Solution:
Remove the "force" flag and always close float windows as if :close! is
used when closing the last window of a tabpage, and set the "free_buf"
flag for a float window based on whether its buffer can be freed.
As 'hidden' is on by default, this change shouldn't affect many people.

src/nvim/api/window.c

@@ -370,7 +370,7 @@ void nvim_win_hide(Window window, Error *err)
   TryState tstate;
   try_enter(&tstate);
   if (tabpage == curtab) {
-    win_close(win, false, false);
+    win_close(win, false);
   } else {
     win_close_othertab(win, false, tabpage);
   }