Remove reg-id checking in the GRUU plugin

I am working a bit with jssip and in my troubleshooting discovered that GRUU was not working as expected.   Jssip is sending a reg-id (for outbound support) and the existence of this header caused GRUU not to generate GRUU ids.  I did some searching and it appears reg-id is completely optional in terms of GRUU and so I removed the check completely.    Let me know if I missed something here.  

Thanks!