Remove g_sequence_lookup from qemu-img help function

g_sequence_lookup is not supported by glib < 2.28. The usage
of g_sequence_lookup is not essential in this context (it's a
safeguard against duplicate values in the help message).
Removing the call enables the build on all platforms and
does not change the operation of the help function.
Signed-off-by: NMike Day <ncmike@ncultra.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -70,11 +70,8 @@ static void add_format_to_seq(void *opaque, const char *fmt_name)
 {
     GSequence *seq = opaque;
 
-    if (!g_sequence_lookup(seq, (gpointer)fmt_name,
-                           compare_data, NULL)) {
     g_sequence_insert_sorted(seq, (gpointer)fmt_name,
                              compare_data, NULL);
-    }
 }
 
 static void QEMU_NORETURN GCC_FMT_ATTR(1, 2) error_exit(const char *fmt, ...)