nimble/ll: Remove DTM event from queue before erasing context

If event is already queued, we should remove it from queue before
erasing context as otherwise callback and link will be removed which
leads to a crash when event is run,

nimble/controller/src/ble_ll_dtm.c

@@ -427,6 +427,7 @@ ble_ll_dtm_ctx_free(struct dtm_ctx * ctx)
     OS_EXIT_CRITICAL(sr);
 
     ble_ll_sched_rmv_elem(&ctx->sch);
+    ble_npl_eventq_remove(&g_ble_ll_data.ll_evq, &g_ble_ll_dtm_ctx.evt);
 
     ble_phy_disable();
     ble_phy_disable_dtm();