Prevent CU-UP UE ID management if integrated CU

This is a workaround for a problem described in #706 . In short, we cannot reuse the cu_add/remove_f1_ue_data() functions if we are in an integrated CU-UP/CP ("CU"), but then we can also not reliably use different UE IDs in CU-UP and CU-CP (except if using even more hacks).

This problem will happen with multiple UEs once !2423 (merged) is merged, so let's be proactive and handle that now. I tested with 6 UEs in monolithic/F1/F1+E1 cases, and did not observe the problems anymore.