[autotest] Drop the USB stick check from ServoHost.verify_software().

The check was meant to look out for regressions in an old bug seen
on Ubuntu kernels, chromium:225932.  However, that problem hasn't
been seen in ages, and maintaining code that can _correctly_
find regressions is now causing more problems than it solves.

So, we're deleting the code.

BUG=chrome-os-partner:31799
TEST=None

Change-Id: I3788e45c2f4ee39b9e883f1fc505430ab5051376
Reviewed-on: https://chromium-review.googlesource.com/217499
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Yusuf Mohsinally <mohsinally@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
diff --git a/server/hosts/servo_host.py b/server/hosts/servo_host.py
index 0f58b05..215cba3 100644
--- a/server/hosts/servo_host.py
+++ b/server/hosts/servo_host.py
@@ -315,29 +315,6 @@
             raise ServoHostVerifyFailure(error_msg)
 
 
-    def _check_servo_host_usb(self):
-        """A sanity check of the USB device.
-
-        Test that the USB stick has been properly unplugged.  An old
-        kernel bug sometimes allowed the USB stick block device node
-        to be wedged such that it couldn't be unplugged.
-
-        Servo initialization unplugs the stick, so as a prophylactic
-        against a regression, we check that the USB stick is
-        actually unplugged.  (For reference, see crbug.com/225932.)
-
-        @raises ServoHostVerifyFailure if /dev/sda exists
-
-        """
-        try:
-            # The following test exits with a non-zero code
-            # and raises AutoserverRunError if error is detected.
-            self.run('test ! -b /dev/sda')
-        except (error.AutoservRunError, error.AutoservSSHTimeout) as e:
-            raise ServoHostVerifyFailure(
-                    'USB sanity check on %s failed: %s' % (self.hostname, e))
-
-
     def _check_servo_config(self):
         """Check if config file exists for servod.
 
@@ -516,8 +493,6 @@
                     timeout_sec=self.INITIALIZE_SERVO_TIMEOUT_SECS)
             if timeout:
                 raise ServoHostVerifyFailure('Servo initialize timed out.')
-
-        self._check_servo_host_usb()
         logging.info('Sanity checks pass on servo host %s', self.hostname)