patch 8.0.1728: condition always false, useless code

Problem:    Condition always false, useless code.
Solution:   Remove the code. (Nikolai Pavlov, closes #2808)

Author: brammool

src/message.c

@@ -1703,8 +1703,6 @@ str2special(
 	{
 	    c = TO_SPECIAL(str[1], str[2]);
 	    str += 2;
-	    if (c == KS_ZERO)	/* display <Nul> as ^@ or <Nul> */
-		c = NUL;
 	}
 	if (IS_SPECIAL(c) || modifiers)	/* special key */
 	    special = TRUE;

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1728,
 /**/
     1727,
 /**/