hevc_refs: drop the handling of negative coordinates in ff_hevc_get_ref_list()

It makes no sense and cannot happen in any place from which this
function is called.

libavcodec/hevc_refs.c

@@ -51,16 +51,12 @@ void ff_hevc_unref_frame(HEVCContext *s, HEVCFrame *frame, int flags)
 
 RefPicList *ff_hevc_get_ref_list(HEVCContext *s, HEVCFrame *ref, int x0, int y0)
 {
-    if (x0 < 0 || y0 < 0) {
-        return s->ref->refPicList;
-    } else {
-        int x_cb         = x0 >> s->sps->log2_ctb_size;
-        int y_cb         = y0 >> s->sps->log2_ctb_size;
-        int pic_width_cb = (s->sps->width + (1 << s->sps->log2_ctb_size) - 1) >>
-                           s->sps->log2_ctb_size;
-        int ctb_addr_ts  = s->pps->ctb_addr_rs_to_ts[y_cb * pic_width_cb + x_cb];
-        return (RefPicList *)ref->rpl_tab[ctb_addr_ts];
-    }
+    int x_cb         = x0 >> s->sps->log2_ctb_size;
+    int y_cb         = y0 >> s->sps->log2_ctb_size;
+    int pic_width_cb = (s->sps->width + (1 << s->sps->log2_ctb_size) - 1) >>
+        s->sps->log2_ctb_size;
+    int ctb_addr_ts  = s->pps->ctb_addr_rs_to_ts[y_cb * pic_width_cb + x_cb];
+    return (RefPicList *)ref->rpl_tab[ctb_addr_ts];
 }
 
 void ff_hevc_clear_refs(HEVCContext *s)