Fix to avoid RRC->X2AP ticking every msec when x2ap task is not enabled

-Since the aim of itti RRC_SUBFRAME_PROCESS message (MAC->RRC) is to trigger an itti X2AP_SUBFRAME_PROCESS message (RRC->X2AP), avoid creating
RRC_SUBFRAME_PROCESS message if x2ap task is not enabled. Not sure if this is the best solution. We could also avoid only sending the second itti X2AP_SUBFRAME_PROCESS message.