avcodec/videotoolbox: fix -Wunused-but-set-variable

pix_fmt was declared presumably to shorten the argument passed to the function.
However, it is currently not being used for such a purpose.
This patch simply removes it instead.
This fixes -Wunused-but-set-variable reported at e.g:
http://fate.ffmpeg.org/log.cgi?time=20150919194249&log=compile&slot=x86_64-darwin-gcc-4.9.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/videotoolbox.c

@@ -501,7 +501,6 @@ static int videotoolbox_default_init(AVCodecContext *avctx)
     VTDecompressionOutputCallbackRecord decoder_cb;
     CFDictionaryRef decoder_spec;
     CFDictionaryRef buf_attr;
-    int32_t pix_fmt;
 
     if (!videotoolbox) {
         av_log(avctx, AV_LOG_ERROR, "hwaccel context is not set\n");
@@ -528,8 +527,6 @@ static int videotoolbox_default_init(AVCodecContext *avctx)
         break;
     }
 
-    pix_fmt = videotoolbox->cv_pix_fmt_type;
-
     decoder_spec = videotoolbox_decoder_config_create(videotoolbox->cm_codec_type, avctx);
 
     videotoolbox->cm_fmt_desc = videotoolbox_format_desc_create(videotoolbox->cm_codec_type,