Drop redundant arc4random prototypes from cygwin/stdlib.h

Newlib's default stdlib.h now defines these functions so we can
drop them from the Cygwin-specific header.  Remove the arc4random_stir
and arc4random_uniform prototypes entirely.  They shouldn't be used
by applications anymore.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/include/cygwin/stdlib.h

@@ -19,12 +19,6 @@ extern "C"
 {
 #endif
 
-__uint32_t arc4random(void);
-void arc4random_addrandom(unsigned char *, int);
-void arc4random_buf(void *, size_t);
-void arc4random_stir(void);
-__uint32_t arc4random_uniform(__uint32_t);
-
 const char *getprogname (void);
 void	setprogname (const char *);