ul_harq: don't clear payload buffer during HARQ reset

this fixes #1791 which was caused by a race between
the PHY workers preparing a new UL-SCH PDU and the Stack resetting
the MAC, which in turn resets the HARQ processeses, which cleared
the buffer and set the length to zero.

Not explicitly clearing the buffer on a HARQ reset seems to be
fine since the PHY worker clears the buffer before starting to
pack a new PDU anyway.

srsue/src/stack/mac/ul_harq.cc

@@ -169,7 +169,6 @@ void ul_harq_entity::ul_harq_process::reset()
   current_irv         = 0;
   is_grant_configured = false;
   bzero(&cur_grant, sizeof(mac_interface_phy_lte::mac_grant_ul_t));
-  payload_buffer->clear();
 }
 
 void ul_harq_entity::ul_harq_process::reset_ndi()