unix: remove Getdirentries on iOS

This system call doesn't exist on iOS.

Update golang/go#28984

Change-Id: I92eb6fd3eb263863a31338bc18c9826a2242434b
Reviewed-on: https://go-review.googlesource.com/c/154659
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_darwin.go

@@ -437,7 +437,6 @@ func Uname(uname *Utsname) error {
 //sys	Fpathconf(fd int, name int) (val int, err error)
 //sys	Fsync(fd int) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
-//sys	Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) = SYS_GETDIRENTRIES64
 //sys	Getdtablesize() (size int)
 //sysnb	Getegid() (egid int)
 //sysnb	Geteuid() (uid int)