remove vestiges of ffmpeg addition build_index

The change to ffmpeg does nothing and is being removed to reduce MythTV's
downstream changes to FFmpeg, hopefully culminating in an unmodified FFmpeg.

avformatdecoder.cpp probably needs more cleanup, but this is the minimal change
required to enable the removal of build_index from FFmpeg.

mythtv/libs/libmythtv/decoders/avformatdecoder.cpp

@@ -1247,11 +1247,7 @@ int AvFormatDecoder::OpenFile(MythMediaBuffer *Buffer, bool novideo,
         }
 
         m_dontSyncPositionMap = true;
-        m_ic->build_index = 1;
     }
-    // we have a position map, disable libavformat's seek index
-    else
-        m_ic->build_index = 0;
 
     av_dump_format(m_ic, 0, filename, 0);