cleanup: fix redundant NOT_USED macro redefinition (re: 733f70e9)

The NOT_USED() macro is already defined in ast.h (which is included
by shell.h) as an alias of NoP(). So it's better to apply the fix
to NoP() so it takes effect for both verrsions, for libast and ksh.

src/cmd/ksh93/include/shell.h

@@ -37,18 +37,6 @@
 #   include	<nval.h>
 #endif /* _SH_PRIVATE */
 
-/*
- * This is a macro that can be used to silence "unused parameter" warnings from the compiler for
- * functions which need to accept parameters they do not use because they need to be compatible
- * with an interface.
- */
-#undef NOT_USED
-#define NOT_USED(expr)		\
-	do {			\
-		(void)(expr);	\
-	} while (0)
-
-
 /* options */
 typedef struct
 {

src/lib/libast/include/ast.h

@@ -228,7 +228,7 @@ typedef struct
 
 #if defined(__STDC__) || defined(__cplusplus) || defined(c_plusplus)
 #define NiL		0
-#define NoP(x)		(void)(x)
+#define NoP(x)		do (void)(x); while(0)	/* for silencing "unused parameter" warnings */
 #else
 #define NiL		((char*)0)
 #define NoP(x)		(&x,1)