Fix redundant initialization of allow_ref_frame_mvs

The variable oxcf->allow_ref_frame_mvs is initialized twice in the
function set_encoder_config(). This CL removes one of the
initialization, as it is being overwritten.

BUG=aomedia:2701

Change-Id: I0cbe1cbfa2c543cafd127824500c68ba87b4a752
diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 132920f..0d59ae2 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -829,7 +829,6 @@
       (cfg->g_usage == AOM_USAGE_REALTIME) ? 0 : extra_cfg->enable_restoration;
   oxcf->force_video_mode = extra_cfg->force_video_mode;
   oxcf->enable_palette = extra_cfg->enable_palette;
-  oxcf->allow_ref_frame_mvs = extra_cfg->enable_ref_frame_mvs;
 
   // Set Quantization related configuration.
   q_cfg->using_qm = extra_cfg->enable_qm;