Revert "unix: implement SyncFileRange for ARM"

This reverts commit 1b73967140
(https://go-review.googlesource.com/c/sys/+/133015)

Reason for revert: breaks the build. No Syscall9 on linux/arm.
Not obvious this was ever compiled, much less tested. (Unless it
was only for gccgo, but that was never specified.)

Fixes golang/go#27475

Change-Id: I5ed04c9ab672ce33adc64bdaec601bf73df42269
Reviewed-on: https://go-review.googlesource.com/133095
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_arm.go

@@ -141,7 +141,6 @@ func Fadvise(fd int, offset int64, length int64, advice int) (err error) {
 }
 
 //sys	mmap2(addr uintptr, length uintptr, prot int, flags int, fd int, pageOffset uintptr) (xaddr uintptr, err error)
-//sys	SyncFileRange(fd int, off int64, n int64, flags int) (err error) = SYS_ARM_SYNC_FILE_RANGE
 
 func Fstatfs(fd int, buf *Statfs_t) (err error) {
 	_, _, e := Syscall(SYS_FSTATFS64, uintptr(fd), unsafe.Sizeof(*buf), uintptr(unsafe.Pointer(buf)))