nhrp: Make sure `no ip nhrp map <something>` works as expected

nhrp: Make sure `no ip nhrp map <something>` works as expected

We passed peer as NULL and nothing happened.

exit2-debian-9# conf
exit2-debian-9(config)# int gre1
exit2-debian-9(config-if)# ip nhrp map 1.1.1.1 local
exit2-debian-9(config-if)# ip nhrp map 2.2.2.2 3.3.3.3
exit2-debian-9(config-if)# do sh run
...
!
interface gre1
 ip nhrp map 1.1.1.1 local
 ip nhrp map 2.2.2.2 3.3.3.3
!
...
exit2-debian-9(config-if)# no ip nhrp map 1.1.1.1
exit2-debian-9(config-if)# do sh run
...
!
interface gre1
 ip nhrp map 2.2.2.2 3.3.3.3
!

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>