Reviewer Andy: Remove extra AVPs from I-CSCF and BGCF ACRs

Andy

Can you review my fix to https://github.com/Metaswitch/sprout/issues/500.  This should be along the lines of what we discussed last night, although when coding I found a few other cases where we were including AVPs in cases we shouldn't.  I've tested the main issues by adding a UT case for I-CSCF REGISTER handling.

I decided to leave the Server-Capabilities AVP in the final ACR reported by the I-CSCF as it feels like this is useful and not confusing.

Mike