avconv: remove unused variable opt_shortest

This was replaced with a per-file value in 3c0df905.
Signed-off-by: Mans Rullgard <mans@mansr.com>

avconv_opt.c

@@ -69,7 +69,6 @@ int do_hex_dump       = 0;
 int do_pkt_dump       = 0;
 int copy_ts           = 0;
 int copy_tb           = 1;
-int opt_shortest      = 0;
 int exit_on_error     = 0;
 int print_stats       = 1;
 int qp_hist           = 0;