enb_proxy: obtain sctp_client sockopts from the env directly
Instead of passing the MmeConnCfg (sctp_client:cfg()) all the way from osmo_s1gw_sup through sctp_server (as priv) into enb_proxy (as state), read the sctp_client configuration in-place via osmo_s1gw:get_env/2 when it is actually needed (connecting/enter).
This works because osmo_s1gw_sup already normalizes and writes back the complete sctp_client config to the application env (set_env/2) before starting the supervision tree.
As a result, mme_conn_cfg is removed from enb_proxy's state record, start_link/2 no longer uses its Priv argument, and server_cfg/1 is simplified to server_cfg/0.
