avdevice/avfoundation: remove write-only variable

The block_buffer was only ever written to but then never used in the
following code, making it unnecessary.

Fixes a "variable 'block_buffer' set but not used" compiler warning.

libavdevice/avfoundation.m

@@ -632,7 +632,6 @@ static int get_video_config(AVFormatContext *s)
 {
     AVFContext *ctx = (AVFContext*)s->priv_data;
     CVImageBufferRef image_buffer;
-    CMBlockBufferRef block_buffer;
     CGSize image_buffer_size;
     AVStream* stream = avformat_new_stream(s, NULL);
 
@@ -652,7 +651,6 @@ static int get_video_config(AVFormatContext *s)
     avpriv_set_pts_info(stream, 64, 1, avf_time_base);
 
     image_buffer = CMSampleBufferGetImageBuffer(ctx->current_frame);
-    block_buffer = CMSampleBufferGetDataBuffer(ctx->current_frame);
 
     if (image_buffer) {
         image_buffer_size = CVImageBufferGetEncodedSize(image_buffer);