stp: fix TC_rkm_unreg_active

Create the routing key dynamically, before trying to unregister it, so
osmo-stp doesn't answer with ERR_NOT_REG instead of the expected
ERR_ASP_ACTIVE. While at it, add missing clean up logic.

Related: OS#4239
Change-Id: I31fcba85d23a8767eb0ceb513ff5b440558a475b
1 file changed