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;

src/stdio/fflush.c

@@ -15,9 +15,6 @@ static int __fflush_unlocked(FILE *f)
 	f->wpos = f->wbase = f->wend = 0;
 	f->rpos = f->rend = 0;
 
-	/* Hook for special behavior on flush */
-	if (f->flush) f->flush(f);
-
 	return 0;
 }
 

src/stdio/freopen.c

@@ -32,7 +32,6 @@ FILE *freopen(const char *filename, const char *mode, FILE *f)
 	f->write = f2->write;
 	f->seek = f2->seek;
 	f->close = f2->close;
-	f->flush = f2->flush;
 
 	fclose(f2);
 	return f;