refactor: deprecate 'secure' option

Now that 'exrc' files must be explicitly marked trusted there is no need
to constrain what can be done in them.

runtime/doc/options.txt

@@ -5129,19 +5129,6 @@ A jump table for the options with a short description can be found at |Q_op|.
 	two letters (See |object-motions|).  The default makes a section start
 	at the nroff macros ".SH", ".NH", ".H", ".HU", ".nh" and ".sh".
 
-						*'secure'* *'nosecure'* *E523*
-'secure'		boolean	(default off)
-			global
-	When on, ":autocmd", shell and write commands are not allowed in
-	".nvimrc" and ".exrc" in the current directory and map commands are
-	displayed.  Switch it off only if you know that you will not run into
-	problems, or when the 'exrc' option is off.  On Unix this option is
-	only used if the ".nvimrc" or ".exrc" is not owned by you.  This can be
-	dangerous if the systems allows users to do a "chown".  You better set
-	'secure' at the end of your |init.vim| then.
-	This option cannot be set from a |modeline| or in the |sandbox|, for
-	security reasons.
-
 						*'selection'* *'sel'*
 'selection' 'sel'	string	(default "inclusive")
 			global

runtime/doc/vim_diff.txt

@@ -641,6 +641,9 @@ Options:
   *'prompt'* *'noprompt'*
   *'remap'* *'noremap'*
   *'restorescreen'* *'rs'* *'norestorescreen'* *'nors'*
+  *'secure'*
+    Everything is allowed in 'exrc' files since they must be explicitly marked
+    trusted.
   *'shelltype'*
   *'shortname'* *'sn'* *'noshortname'* *'nosn'*
   *'swapsync'* *'sws'*