search: remove legacy 'search[sort]' params.

Remove support for the `search[sort]` param on certain index pages. This
hasn't been used for years, and it caused the `search[order]=` param to
be added to pagination links even when the order was blank.

app/models/pool.rb

@@ -70,7 +70,6 @@ class Pool < ApplicationRecord
         q = q.collection
       end
 
-      params[:order] ||= params.delete(:sort)
       case params[:order]
       when "name"
         q = q.order("pools.name")