cdef: remove AOM_UNUSED

cm is always used because high bitdepth is enabled by default.

Change-Id: I71e3c0681c7b99b7c56faebb40628d96dd4393a7
diff --git a/av1/common/cdef.c b/av1/common/cdef.c
index e9e2b0e..97f250d 100644
--- a/av1/common/cdef.c
+++ b/av1/common/cdef.c
@@ -107,7 +107,7 @@
   }
 }
 
-static void copy_sb8_16(AOM_UNUSED AV1_COMMON *cm, uint16_t *dst, int dstride,
+static void copy_sb8_16(AV1_COMMON *cm, uint16_t *dst, int dstride,
                         const uint8_t *src, int src_voffset, int src_hoffset,
                         int sstride, int vsize, int hsize) {
   if (cm->seq_params.use_highbitdepth) {