provider: Discard routes to VPN server via VPN tunnel

provider: Discard routes to VPN server via VPN tunnel

These obsolete routes must be discarded as we cannot
contact VPN server via VPN tunnel. If the VPN server
address is not yet resolved, we do that while starting
the VPN client process.

Example of non working routes:
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         0.0.0.0         0.0.0.0         UG    0      0        0 vpn0
10.10.15.0      0.0.0.0         255.255.255.0   U     0      0        0 wlan0
10.10.15.1      0.0.0.0         255.255.255.255 UH    0      0        0 wlan0
10.252.112.0    0.0.0.0         255.255.240.0   U     0      0        0 vpn0
193.229.0.40    10.10.15.1      255.255.255.255 UGH   0      0        0 wlan0
193.229.0.42    10.10.15.1      255.255.255.255 UGH   0      0        0 wlan0
113.108.211.91  0.0.0.0         255.255.255.255 UH    0      0        0 vpn0
113.108.211.91  10.10.15.1      255.255.255.255 UGH   0      0        0 wlan0

The correct routes should be like this:
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         0.0.0.0         0.0.0.0         U     0      0        0 vpn0
10.10.15.0      0.0.0.0         255.255.255.0   U     0      0        0 wlan0
10.10.15.1      0.0.0.0         255.255.255.255 UH    0      0        0 wlan0
10.252.112.0    0.0.0.0         255.255.240.0   U     0      0        0 vpn0
193.229.0.40    10.10.15.1      255.255.255.255 UGH   0      0        0 wlan0
193.229.0.42    10.10.15.1      255.255.255.255 UGH   0      0        0 wlan0
113.108.211.91  10.10.15.1      255.255.255.255 UGH   0      0        0 wlan0