danger: Stop complaining when MR has no labels

Since Danger now tries to apply "category" labels based on the diff,
it almost never happen that MR has no labels, and since this check looks
at the labels currently set on the MR, it often complains even if Danger
adds "category" labels in the same process, which is annoying.
Signed-off-by: Rémy Coutable <remy@rymai.me>

danger/metadata/Dangerfile

@@ -17,10 +17,6 @@ if gitlab.mr_body.size < 5
   fail "Please provide a proper merge request description."
 end
 
-if gitlab.mr_labels.empty?
-  fail "Please add labels to this merge request."
-end
-
 if (THROUGHPUT_LABELS & gitlab.mr_labels).empty?
   warn 'Please add a [throughput label](https://about.gitlab.com/handbook/engineering/management/throughput/#implementation) to this merge request.'
 end