Remove unnecessary class variable

This variable is only used  by one testcase

Change-Id: I7615fa389de42239f8ec6ed724604b230df0354f
diff --git a/tempest/api/volume/test_volumes_negative.py b/tempest/api/volume/test_volumes_negative.py
index 7d745f2..f139283 100644
--- a/tempest/api/volume/test_volumes_negative.py
+++ b/tempest/api/volume/test_volumes_negative.py
@@ -35,7 +35,6 @@
 
         # Create a test shared instance and volume for attach/detach tests
         cls.volume = cls.create_volume()
-        cls.mountpoint = "/dev/vdc"
 
     def create_image(self):
         # Create image
@@ -176,7 +175,7 @@
                           self.volumes_client.attach_volume,
                           data_utils.rand_uuid(),
                           instance_uuid=server['id'],
-                          mountpoint=self.mountpoint)
+                          mountpoint="/dev/vdc")
 
     @decorators.attr(type=['negative'])
     @decorators.idempotent_id('9f9c24e4-011d-46b5-b992-952140ce237a')