sysmobts VTY: update to new libosmocore
sysmobts VTY: update to new libosmocore

libosmocore 40832fcfb58c8c97c66e098c5705352ac5beea8e and later contain
the vty_cmd_string_from_valstr() function, so we shouldn't have
a local / deprecated copy anymore.

telenoobie/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts