Remove g_sequence_lookup from qemu-img help function

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: Mike Day <ncmike@ncultra.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>