tipc: delete tipc_mtu_bad from tipc_udp_enable

Since commit a4dfa72d ("tipc: set default MTU for UDP media"), it's
been no longer using dev->mtu for b->mtu, and the issue described in
commit 3de81b75

 ("tipc: check minimum bearer MTU") doesn't exist
in UDP bearer any more.

Besides, dev->mtu can still be changed to a too small mtu after the UDP
bearer is created even with tipc_mtu_bad() check in tipc_udp_enable().
Note that NETDEV_CHANGEMTU event processing in tipc_l2_device_event()
doesn't really work for UDP bearer.

So this patch deletes the unnecessary tipc_mtu_bad from tipc_udp_enable.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Link: https://lore.kernel.org/r/282f1f5cc40e6cad385aa1c60569e6c5b70e2fb3.1685371933.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>