Remove sync call in md5 data integrity tests

This is called in a section that will
immediately unmount the FS as the next
operation, which will sync that FS's data.
Remove this to save an unneeded ssh call.

Change-Id: I9d1317c31575eabd89c3cbc7f6b5459409a1854b
diff --git a/cinder_tempest_plugin/scenario/manager.py b/cinder_tempest_plugin/scenario/manager.py
index 8598ade..cffa044 100644
--- a/cinder_tempest_plugin/scenario/manager.py
+++ b/cinder_tempest_plugin/scenario/manager.py
@@ -110,7 +110,6 @@
                 (mount_path, filename))
             md5 = ssh_client.exec_command(
                 'sudo md5sum -b %s/%s|cut -c 1-32' % (mount_path, filename))
-            ssh_client.exec_command('sudo sync')
         return md5
 
     def get_md5_from_file(self, instance, instance_ip, filename,