plugin/nec: Drop description field from PFC create_tenant

Some version of PFC, one of OpenFlow controllers supported in NEC
plugin, does not accept "description" field in create_tenant().

Removes update_tenant() in pfc.py since it is not used anywhere.

Fixes bug #1166076

Change-Id: I817bb67465ff47df43ad2ea866e40c4413f29cbb