[tracer] remove useless line, pron to crashes

Setting the tracing info into the processing context here is useless as none of the following funcions are using /accessing this context.
Even more, there are cases (like tracing an UAC b2b transaction triggered by a b2b timeout) where there is not processing context set (so crashing).

modules/tracer/tracer.c

@@ -1427,9 +1427,6 @@ static int trace_b2b_transaction(struct sip_msg* msg, void *trans, void* param)
 	trace_info_p info = (trace_info_p)param;
 	struct cell *t = (struct cell*)trans;
 
-	/* context for the request message */
-	SET_TRACER_CONTEXT( info );
-
 	if (t==T_UNDEFINED) {
 		/* Negative hop-by-hop ACK shouldn't be here */
 		LM_BUG("undefined transaction received here\n");