versions: remove 'subsequent' version comparison option.

This option was rarely used and what it actually did was usually
difficult to understand.

app/helpers/application_helper.rb

@@ -62,7 +62,7 @@ module ApplicationHelper
 
   def version_type_links(params)
     html = []
-    %w[previous subsequent current].each do |type|
+    %w[previous current].each do |type|
       if type == params[:type]
         html << %{<span>#{type}</span>}
       else

app/helpers/artist_versions_helper.rb

@@ -6,8 +6,6 @@ module ArtistVersionsHelper
     this_names = artist_version.other_names
     if other.present?
       other_names = other.other_names
-    elsif type == "subsequent"
-      other_names = this_names
     else
       other_names = []
     end
@@ -24,8 +22,6 @@ module ArtistVersionsHelper
     this_urls = artist_version.urls
     if other.present?
       other_urls = other.urls
-    elsif type == "subsequent"
-      other_urls = this_urls
     else
       other_urls = []
     end