removed unused databus functions

supervisor/databus.lua

@@ -172,11 +172,4 @@ function databus.tx_pdg_rtt(session_id, rtt)
     end
 end
 
--- link a function to receive data from the bus
----@param field string field name
----@param func function function to link
-function databus.rx_field(field, func)
-    databus.ps.subscribe(field, func)
-end
-
 return databus