filter_editions doesn't support multiple filters

I added this "temporary" warning back in May last year, but I forgot to
make it an error until just now.

The code processing the filters is:

    filters.each do |link_type, target_content_id|
      scope = scope.where(links: { link_type:, target_content_id: })
    end

... so the .where() statements would be ANDed together. If you provided
multiple different filters, you'd never get any results.

I've checked the logs in production, and my warning has not been served
in the last 14 days (our log retention), so I'm pretty confident it's
safe to tighten this up.

app/models/link.rb

@@ -14,12 +14,7 @@ class Link < ApplicationRecord
   validate :association_presence
 
   def self.filter_editions(scope, filters)
-    if filters.size > 1
-      # TODO: richard.towers - temporary warning to check whether this method is ever
-      #       called with multiple filters. The code looks wrong, so if it's not being
-      #       called we should make this an error and only support a single filter.
-      logger.warn("filter_editions called with multiple filters. These will be ANDed together in a way that probably isn't what we want. Filters were: #{filters.inspect}")
-    end
+    raise "filter_editions doesn't support multiple filters" if filters.size > 1
 
     scope = scope.joins(document: :link_set_links)