FAFT: Remove backup directory which is no longer used

This backup directory is created tempoarily to store the firmware
image. After sending the file to server, the directory is no longer
used and should be removed.

BUG=None
TEST=Ran a FAFT test, verified the backup directory was removed.

Change-Id: I3a1007a7d32cf92f415a6dd221e0492e4b7911be
Reviewed-on: https://chromium-review.googlesource.com/295566
Commit-Ready: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Shelley Chen <shchen@chromium.org>
diff --git a/server/cros/faft/firmware_test.py b/server/cros/faft/firmware_test.py
index 8b1ab6d..3f00941 100644
--- a/server/cros/faft/firmware_test.py
+++ b/server/cros/faft/firmware_test.py
@@ -1166,14 +1166,16 @@
         @param suffix: a string appended to backup file name
         """
         remote_temp_dir = self.faft_client.system.create_temp_dir()
-        self.faft_client.bios.dump_whole(os.path.join(remote_temp_dir, 'bios'))
-        self._client.get_file(os.path.join(remote_temp_dir, 'bios'),
+        remote_bios_path = os.path.join(remote_temp_dir, 'bios')
+        self.faft_client.bios.dump_whole(remote_bios_path)
+        self._client.get_file(remote_bios_path,
                               os.path.join(self.resultsdir, 'bios' + suffix))
-
-        self._backup_firmware_sha = self.get_current_firmware_sha()
+        self._client.run('rm -rf %s' % remote_temp_dir)
         logging.info('Backup firmware stored in %s with suffix %s',
             self.resultsdir, suffix)
 
+        self._backup_firmware_sha = self.get_current_firmware_sha()
+
     def is_firmware_saved(self):
         """Check if a firmware saved (called backup_firmware before).