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.

src/nvim/options.lua

@@ -2007,7 +2007,7 @@ return {
     },
     {
       full_name='secure',
-      short_desc=N_("mode for reading .vimrc in current dir"),
+      short_desc=N_("No description"),
       type='bool', scope={'global'},
       secure=true,
       varname='p_secure',