Changes

hnbgw_cn: Remove assert hit due to wrong assumption

The HNBGW in correctly assumes that no ss7->sccp instance is allocated
until the same function calls osmo_sccp_simple_client_on_ss7_id().
This assumption is wrong, since ss7 may create its own ss7->sccp
instance internally as a result of vty configuration, eg. when "sccp
max-optional-data 124" is placed in osmo-hnbgw.cfg file.

In this scenario, simply removing the assert is enough, since
osmo_sccp_simple_client_on_ss7_id() just calls osmo_ss7_ensure_sccp(),
the same that the libmoso-sccp code called to allocate the pointer.

Related: SYS#6566
Fixes: f3caea850bc892551cd09bbaf49f798c172590c4
Change-Id: I1221c165156e9625324cf0080836a8ed2bad4e9c
(cherry picked from commit 8514b73bff61c26d40552a82278317565d6cf110)