ML2 Cisco Nexus MD: Remove unnecessary Cisco nexus DB

The nexus switch information is retrieved from a dictionary created
at initialization. The ML2 cisco credentials database in not needed.

Change-Id: I723293de9f2b821a011db2ad9176a945fef72ee0
Closes-Bug: 1284676
Merge "ML2 Cisco Nexus MD: Remove unnecessary Cisco nexus DB"