Revert "avcodec/hevc: fix outputted AVFrame.key_frame"

This is not correct

Requested-by: mraulet
This reverts commit 973de9eb.

libavcodec/hevc_refs.c

@@ -207,9 +207,6 @@ int ff_hevc_output_frame(HEVCContext *s, AVFrame *out, int flush)
                           (frame->window.top_offset   >> vshift) * dst->linesize[i];
                 dst->data[i] += off;
             }
-
-            out->key_frame = out->pict_type == AV_PICTURE_TYPE_I;
-
             av_log(s->avctx, AV_LOG_DEBUG,
                    "Output frame with POC %d.\n", frame->poc);
             return 1;