bgpd: Don't increment twice notification sent stats counter

bgpd: Don't increment twice notification sent stats counter

Before:

```
$ vtysh -c 'clear ip bgp 192.168.10.17'
$ vtysh -c 'show bgp neighbor 192.168.10.17 json' | jq '."192.168.10.17".messageStats.notificationsSent'
2
```

After:

```
$ vtysh -c 'clear ip bgp 192.168.10.17'
$ vtysh -c 'show bgp neighbor 192.168.10.17 json' | jq '."192.168.10.17".messageStats.notificationsSent'
1
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>