Devices: set interface placement does not remove the old interface placement

Devices: set interface placement does not remove the old interface placement

The command set interface placement is supposed to remove the existing
interface placement and add a new interface placement based on the given
options. My quick test for the CLI shows that the old interface placement
continues to exist. But the new interface placement is also added.

The bug exists in vnet_device_input_unassign_thread which checks
the old thread index is the same as the passed thread index and skips the
deletion if they are the same.

The fix is to remove the check which is not supposed to be there.

Change-Id: Ib055721fad47513949a03b3cb6dc292bd19fd1e8
Signed-off-by: Steven <sluong@cisco.com>