ryu/lib/packet/lldp.py: remove __str__ method from class LLDPBasicTLV and class lldp

Those methods are legacy when lldp.py was written for dpkt.
They don't make sense. remove them.

Cc: "can." <can@canx.me>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
lib/packet/lldp.py: pylint

> W: 78,0:LLDPBasicTLV.__init__: Unused argument 'args'
> W: 78,0:LLDPBasicTLV.__init__: Unused argument 'kwargs'
> W:109,4:lldp.__init__: __init__ method from base class 'PacketBase' is not called

Cc: "can." <can@canx.me>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>