gluon-client-bridge: simplify OWE-TM configuration (#2550)

OpenWrt now allows to specify the ifname of the transition interface
instead of SSID and BSSID, internally automatically detecting these from
interfaces on the same PHY. Thus, these cross-VAP dependant
configuration can be omitted from UCI.

Signed-off-by: David Bauer <mail@david-bauer.net>