fix misleading use of _POSIX_VDISABLE in sys/ttydefaults.h

_POSIX_VDISABLE is only visible if unistd.h has already been included,
so conditional use of it here makes no sense. the value is always 0
anyway; it does not vary.

include/sys/ttydefaults.h

@@ -9,13 +9,8 @@
 #define CTRL(x) ((x)&037)
 #define CEOF CTRL('d')
 
-#ifdef _POSIX_VDISABLE
-#define CEOL _POSIX_VDISABLE
-#define CSTATUS _POSIX_VDISABLE
-#else
 #define CEOL '\0'
 #define CSTATUS '\0'
-#endif
 
 #define CERASE 0177
 #define CINTR CTRL('c')