[VNET] Fail to remove VNET interface

[volodymyrsamotiy]

Description
VNET interface cannot be removed due to reference count is equal to 1.
It happens because there is no event from config manager for removing subnet route from the interface. As a result, subnet route is not remove hence interface is still in use and cannot be removed.

ERR swss#orchagent: :- meta_generic_validation_remove: object 0x60000000006bd reference count is 1, can't remove
ERR swss#orchagent: :- removeRouterIntfs: Failed to remove router interface for port Vlan3001, rv:-5
INFO swss#supervisord: orchagent terminate called after throwing an instance of 'std::runtime_error'
INFO swss#supervisord: orchagent   what():  Failed to remove router interface.
INFO swss#supervisord INFO exited: orchagent (terminated by SIGABRT (core dumped); not expected)
INFO swss#supervisord WARN received SIGTERM indicating exit request

Steps to reproduce

• Create VNET and VNET interface
• Remove interface
• Observe error "Failed to remove router interface"

Expected behavior
There is create subnet route event for VNET interface from config manager to vnetorch so it is correctly added to HW, but there is now remove subnet route in order to remove it from HW. Expected is that config manager sends remove event for VNET subnet route as well so it will be removed from HW.

[prsunny]

Fixes as part of - #1225