BURs: remove ability to skip secondary validations.

Remove the ability to skip secondary validations when creating a BUR.
The only skippable validation that still existed was the requirement
that both tags in an implication must have wiki pages. It's now
mandatory to write wiki pages for tags before you can request an
implication. This doesn't apply to empty tags.

test/factories/bulk_update_request.rb

@@ -4,6 +4,5 @@ FactoryBot.define do
     title {"xxx"}
     script {"create alias aaa -> bbb"}
     reason { FFaker::Lorem.sentences.join(" ") }
-    skip_secondary_validations {true}
   end
 end