Remove pad in aom_codec_cx_pkt_t data

the size of aom_codec_cx_pkt_t is already bigger than 128 bytes.

This is inherited from libvpx, where vpx_codec_cx_pkt_t also exceeds 128
bytes. So this change could be backported to libvpx.

Bug: aomedia:2980
Change-Id: Ib15aefcb14d365ff245c37cad2d5917e9e0853a2
(cherry picked from commit 8d7db2365f902cf2a2f0012fb2e33cf2bbdd3cc9)

aom/aom_encoder.h

diff --git a/aom/aom_encoder.h b/aom/aom_encoder.h
index a8b39ed..e6aa95f 100644
--- a/aom/aom_encoder.h
+++ b/aom/aom_encoder.h
@@ -142,15 +142,8 @@
       double psnr_hbd[4];
     } psnr;              /**< data for PSNR packet */
     aom_fixed_buf_t raw; /**< data for arbitrary packets */
-
-    /* This packet size is fixed to allow codecs to extend this
-     * interface without having to manage storage for raw packets,
-     * i.e., if it's smaller than 128 bytes, you can store in the
-     * packet list directly.
-     */
-    char pad[128 - sizeof(enum aom_codec_cx_pkt_kind)]; /**< fixed sz */
-  } data;                                               /**< packet data */
-} aom_codec_cx_pkt_t; /**< alias for struct aom_codec_cx_pkt */
+  } data;                /**< packet data */
+} aom_codec_cx_pkt_t;    /**< alias for struct aom_codec_cx_pkt */
 
 /*!\brief Rational Number
  *