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/help.c

@@ -220,7 +220,7 @@ void ex_helpclose(exarg_T *eap)
 {
   FOR_ALL_WINDOWS_IN_TAB(win, curtab) {
     if (bt_help(win->w_buffer)) {
-      win_close(win, false, eap->forceit);
+      win_close(win, false);
       return;
     }
   }