ppc: don't define bamboo-0.13 as the default machine

It has been broken by commit 977b6b91cee1132f8c7b12d22f4b273091598e44.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/hw/ppc440_bamboo.c b/hw/ppc440_bamboo.c
index 8a50836..efe5a77 100644
--- a/hw/ppc440_bamboo.c
+++ b/hw/ppc440_bamboo.c
@@ -177,7 +177,6 @@
     .alias = "bamboo",
     .desc = "bamboo",
     .init = bamboo_init,
-    .is_default = 1,
 };
 
 static QEMUMachine bamboo_machine_v0_12 = {