In 536b892c I've made a thinko: the documentation on Amarisoft says(*) that
if those parameters are not given, their values are taken from the
"serving cell". With that and after reading https://www.telecomhall.net/t/changing-allowedmeasbandwidth/13045,
especially https://www.telecomhall.net/t/changing-allowedmeasbandwidth/13045/5 I've
made the conclusion that if allowed_meas_bandwidth, and similarly
antenna_port_1, are not provided, they will be extracted by UE from MIB
of that peer serving cell.

But I was wrong as eNB does not start if allowed_meas_bandwidth is not
given for eautra-kind handover in NR cell.

So after checking things in 36.331 we see that:

allowed_meas_bandwidth is optional only in SIB3 (Intra Frequency Cell
Reselection), but in SIB5 (Inter Frequency Neighbour Cell) it is
mandatory, and that is the reason for why enb was complaining about that
missing for NR->LTE handover.

-> restore allowed_meas_bandwidth and antenna_port_1 in HO config.

For intra-eNB HO things are clear, but for inter-eNB HO use conservative
values and put a TODO for considering to extend peer/cell/lte schema
with .allowed_meas_bandwidth and .antenna_port_1 . I think keeping the
schemas minimal is better for now until we really need to do inter-RAT
HO in practice.

(*) https://tech-academy.amarisoft.com/lteenb.doc#prop.ncell_list.allowed_meas_bandwidth