use O_DIRECT for all ReadFileStream (#13324)

This PR also removes #13312 to ensure
that we can use a better mechanism to
handle page-cache, using O_DIRECT
even for Range GETs.

internal/disk/fdatasync_unix.go

@@ -30,14 +30,7 @@ func Fdatasync(f *os.File) error {
 	return syscall.Fsync(int(f.Fd()))
 }
 
-// fdavise advice constants
-const (
-	FadvSequential = 0
-	FadvNoReuse    = 0
-)
-
-// Fadvise implements possibility of choosing
-// offset: 0, length: 0
-func Fadvise(f *os.File, advice int) error {
+// FadviseDontNeed is a no-op
+func FadviseDontNeed(f *os.File) error {
 	return nil
 }