Schedule SRS longer in advance, cleanup

Schedule SRS longer in advance, so that it would also be enough even
with big k2 for UL TDA.

Remove useless types: we do not use the information on sched_srs
anymore, so remove it.

Check that the VRB map is free before making allocations.

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_primitives.c

@@ -2766,8 +2766,6 @@ bool add_connected_nr_ue(gNB_MAC_INST *nr_mac, NR_UE_info_t *UE)
 
   NR_UE_sched_ctrl_t *sched_ctrl = &UE->UE_sched_ctrl;
   sched_ctrl->dl_max_mcs = 28; /* do not limit MCS for individual UEs */
-  sched_ctrl->sched_srs.frame = -1;
-  sched_ctrl->sched_srs.slot = -1;
   sched_ctrl->pdcch_cl_adjust = 0;
   reset_srs_stats(UE);