migration: remove check against colo support

Since commit a15215f3 ("build: remove --enable-colo/--disable-colo"),
colo is always supported. We don't need any colo_supported() now since
it is always true. Removing any extra code that depends on it.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Hailiang Zhang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Zhang Chen<zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1500349150-13240-8-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
