Do not map asf metadata key WM/Track to track.

WM/Track is 0-based (and deprecated in favour of WM/TrackNumber),
track is 1-based.

Fixes ticket #825.

libavformat/asf.c

@@ -152,7 +152,6 @@ const AVMetadataConv ff_asf_metadata_conv[] = {
     { "WM/Publisher"       , "publisher"   },
     { "WM/Tool"            , "encoder"     },
     { "WM/TrackNumber"     , "track"       },
-    { "WM/Track"           , "track"       },
     { "WM/MediaStationCallSign", "service_provider" },
     { "WM/MediaStationName", "service_name" },
 //  { "Year"               , "date"        }, TODO: conversion year<->date