Remove support for CanDelete() API from AgentDBEntry

In VGW case, a VRF can be added by IFMAP config and also by VGW
configuration. The VRF entry should not be deleted until both the config
are deleted. CanDelete() API was added in AgentDBEntry to skip delete of
VRF entry until both IFMap and VGW config are deleted.

DB infastructure now supports similar functionality by returning "false"
from DBTable::Delete() API. Remove CanDelete() and use the new DB
infrastructure

Change-Id: Ibe1332f572b819273f7eb3238941a561956c03b7
Merge "Remove support for CanDelete() API from AgentDBEntry"