provider/aws: remove default from associate_public_ip_address

I snuck this in with #2263 because thought it was simply a stylistic
clarity thing, but it actually generates a resource-replacement-forcing
diff for existing resources that don't have this set in the config.
Definitely don't want that. :P

/cc @catsby

builtin/providers/aws/resource_aws_instance.go

@@ -38,7 +38,6 @@ func resourceAwsInstance() *schema.Resource {
 
 			"associate_public_ip_address": &schema.Schema{
 				Type:     schema.TypeBool,
-				Default:  false,
 				ForceNew: true,
 				Optional: true,
 			},