refactor(mch): last mch_ function/macro hits the dust

Also remove some stray comments.

src/nvim/bufwrite.c

@@ -1402,7 +1402,7 @@ int buf_write(buf_T *buf, char *fname, char *sfname, linenr_T start, linenr_T en
       while ((fd = os_open(wfname, fflags, mode)) < 0) {
         // A forced write will try to create a new file if the old one
         // is still readonly. This may also happen when the directory
-        // is read-only. In that case the mch_remove() will fail.
+        // is read-only. In that case the os_remove() will fail.
         if (err.msg == NULL) {
 #ifdef UNIX
           FileInfo file_info;

src/nvim/macros.h

@@ -82,15 +82,6 @@
 #define READBIN    "rb"
 #define APPENDBIN  "ab"
 
-// mch_open_rw(): invoke os_open() with third argument for user R/W.
-#if defined(UNIX)  // open in rw------- mode
-# define MCH_OPEN_RW(n, f)      os_open((n), (f), (mode_t)0600)
-#elif defined(MSWIN)
-# define MCH_OPEN_RW(n, f)      os_open((n), (f), S_IREAD | S_IWRITE)
-#else
-# define MCH_OPEN_RW(n, f)      os_open((n), (f), 0)
-#endif
-
 #define REPLACE_NORMAL(s) (((s)& REPLACE_FLAG) && !((s)& VREPLACE_FLAG))
 
 // MB_PTR_ADV(): advance a pointer to the next character, taking care of

src/nvim/memfile.c

@@ -752,7 +752,7 @@ static bool mf_do_open(memfile_T *mfp, char *fname, int flags)
     emsg(_("E300: Swap file already exists (symlink attack?)"));
   } else {
     // try to open the file
-    mfp->mf_fd = MCH_OPEN_RW(mfp->mf_fname, flags | O_NOFOLLOW);
+    mfp->mf_fd = os_open(mfp->mf_fname, flags | O_NOFOLLOW, S_IREAD | S_IWRITE);
   }
 
   // If the file cannot be opened, use memory only

src/nvim/menu.c

@@ -618,8 +618,6 @@ static void free_menu(vimmenu_T **menup)
 {
   vimmenu_T *menu = *menup;
 
-  // Don't change *menup until after calling gui_mch_destroy_menu(). The
-  // MacOS code needs the original structure to properly delete the menu.
   *menup = menu->next;
   xfree(menu->name);
   xfree(menu->dname);

src/nvim/window.c

@@ -5171,7 +5171,6 @@ static void win_free(win_T *wp, tabpage_T *tp)
   alist_unlink(wp->w_alist);
 
   // Don't execute autocommands while the window is halfway being deleted.
-  // gui_mch_destroy_scrollbar() may trigger a FocusGained event.
   block_autocmds();
 
   clear_winopt(&wp->w_onebuf_opt);