rpl_udp: remove destiny init call

This handler is called by auto_init to initialize the destiny
module correctly.

Fix: 'examples/rpl_udp' is currently double initializing destiny.
See: https://github.com/RIOT-OS/RIOT/blob/master/sys/auto_init/auto_init.c#L114
Merge pull request #527 from mehlis/demo-destiny-init

rpl_udp: remove destiny init call