Contrib: avconv was removed from FFmpeg

(cherry picked from commit b87a34a7)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/ffmpeg/rules.mak

@@ -12,7 +12,6 @@ FFMPEGCONF = \
 	--disable-debug \
 	--enable-gpl \
 	--enable-postproc \
-	--disable-avconv \
 	--disable-devices \
 	--disable-protocols \
 	--disable-avfilter \