Disable FAFTSequence._modify_usb_kernel() until it can be made safe.

The method attempts to overwrite local USB storage on the servod
device; however, as written it can't be guaranteed not to overwrite
a block device for critical internal disks on the autotest server.

BUG=chromium-os:38580
TEST=None

Change-Id: I268fa917b9adb93b8571ef0a7af5a16b26f9ff81
Reviewed-on: https://gerrit.chromium.org/gerrit/42589
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index 43ecaee..09e8958 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -1262,26 +1262,7 @@
         Raises:
           error.TestError: if failed to change magic.
         """
-        assert len(from_magic) == 8
-        assert len(to_magic) == 8
-        # USB image only contains one kernel.
-        kernel_part = self._join_part(usb_dev, self.KERNEL_MAP['a'])
-        read_cmd = "sudo dd if=%s bs=8 count=1 2>/dev/null" % kernel_part
-        current_magic = utils.system_output(read_cmd)
-        if current_magic == to_magic:
-            logging.info("The kernel magic is already %s.", current_magic)
-            return
-        if current_magic != from_magic:
-            raise error.TestError("Invalid kernel image on USB: wrong magic.")
-
-        logging.info('Modify the kernel magic in USB, from %s to %s.',
-                     from_magic, to_magic)
-        write_cmd = ("echo -n '%s' | sudo dd of=%s oflag=sync conv=notrunc "
-                     " 2>/dev/null" % (to_magic, kernel_part))
-        utils.system(write_cmd)
-
-        if utils.system_output(read_cmd) != to_magic:
-            raise error.TestError("Failed to write new magic.")
+        raise error.TestError("FAFTSequence._modify_usb_kernel() is unsafe")
 
 
     def corrupt_usb_kernel(self, usb_dev):