remove flush hook cruft that was never used from stdio

there is no need/use for a flush hook. the write function serves this
purpose already. i originally created the hook for implementing mem
streams based on a mistaken reading of posix, and later realized it
wasn't useful but never removed it until now.

src/internal/stdio_impl.h

@@ -57,7 +57,7 @@ struct __FILE_s {
 	int waiters;
 	void *cookie;
 	off_t off;
-	int (*flush)(FILE *);
+	void *dummy4;
 	void *mustbezero_2;
 	unsigned char *shend;
 	off_t shlim, shcnt;