Feat(eos_designs): simplify the RFC5549 logic when separate peer groups are used:

[gmuloc]

          Follow up issue for this PR - simplify the RFC5549 logic when separate peer groups are used:

underlay_rfc5549 , overlay_mlag_rfc_5549	unique peer group (current implementation)	separate peer group
False, False	nothing to do	nothing to do
False, True	not supported	not supported (need to add a guard)
True, False	peer group actiavte ipv6 next-hop, disabled for every neighbor in every VRF	can be simplified as there is no need to disable it anymore for neighbors
True, True	nothing to do	nothing to do

Originally posted by @gmuloc in #4881 (comment)