patch 8.2.0125: :mode no longer works for any system

Problem:    :mode no longer works for any system.
Solution:   Always give an error message.

src/ex_docmd.c

@@ -5927,7 +5927,7 @@ ex_mode(exarg_T *eap)
     if (*eap->arg == NUL)
 	shell_resized();
     else
-	mch_screenmode(eap->arg);
+	emsg(_(e_screenmode));
 }
 
 /*

src/os_amiga.c

@@ -988,16 +988,6 @@ mch_settmode(int tmode)
 	mch_errmsg(_("cannot change console mode ?!\n"));
 }
 
-/*
- * set screen mode, always fails.
- */
-    int
-mch_screenmode(char_u *arg)
-{
-    emsg(_(e_screenmode));
-    return FAIL;
-}
-
 /*
  * Code for this routine came from the following :
  *

src/os_mswin.c

@@ -742,17 +742,6 @@ mch_check_messages(void)
 #endif
 
 
-/*
- * set screen mode, always fails.
- */
-    int
-mch_screenmode(char_u *arg UNUSED)
-{
-    emsg(_(e_screenmode));
-    return FAIL;
-}
-
-
 #if defined(FEAT_LIBCALL) || defined(PROTO)
 /*
  * Call a DLL routine which takes either a string or int param

src/os_unix.c

@@ -3937,16 +3937,6 @@ check_mouse_termcode(void)
     }
 }
 
-/*
- * set screen mode, always fails.
- */
-    int
-mch_screenmode(char_u *arg UNUSED)
-{
-    emsg(_(e_screenmode));
-    return FAIL;
-}
-
 #ifndef VMS
 
 /*

src/proto/os_amiga.pro

@@ -32,7 +32,6 @@ int mch_nodetype(char_u *name);
 void mch_early_init(void);
 void mch_exit(int r);
 void mch_settmode(int tmode);
-int mch_screenmode(char_u *arg);
 int mch_get_shellsize(void);
 void mch_set_shellsize(void);
 void mch_new_shellsize(void);

src/proto/os_mswin.pro

@@ -19,7 +19,6 @@ void display_errors(void);
 int mch_has_exp_wildcard(char_u *p);
 int mch_has_wildcard(char_u *p);
 int mch_chdir(char *path);
-int mch_screenmode(char_u *arg);
 int mch_icon_load(HANDLE *iconp);
 int mch_libcall(char_u *libname, char_u *funcname, char_u *argstring, int argint, char_u **string_result, int *number_result);
 void DumpPutS(const char *psz);

src/proto/os_unix.pro

@@ -54,7 +54,6 @@ int get_tty_info(int fd, ttyinfo_T *info);
 void mch_setmouse(int on);
 void mch_bevalterm_changed(void);
 void check_mouse_termcode(void);
-int mch_screenmode(char_u *arg);
 int mch_get_shellsize(void);
 int mch_report_winsize(int fd, int rows, int cols);
 void mch_set_shellsize(void);

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    125,
 /**/
     124,
 /**/