Table 47 entry not getting removed after SNAT disabled

Issue 1: As part of cleanup, when the external VPN is deleted first, all the NAT external
flows are getting deleted, when the last VM is deleted, it is expected that all
other SNAT related flows should be removed from the NAPT, but this is not happening
due to the null check on external VPN
 
Issue 2: Noticed that the externalVPN metadata flow in table 47 is getting removed even
though there is another router connected to the same externalVPN