Drop the query plan test

Keep the test of what page_updated_since returns, which covers the
rewritten scope; pinning SQLite's plan locks in more than the change
needs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0142qgjggdJ2KDdGk7RF9Xm9

test/models/message/pagination_test.rb

@@ -1,10 +1,6 @@
 require "test_helper"
 
 class Message::PaginationTest < ActiveSupport::TestCase
-  test "messages updated since a time are found through the room and update time index" do
-    assert_match(/USING INDEX index_messages_on_room_id_and_updated_at \(room_id=\? AND updated_at>\?\)/, plan_of_page_updated_since)
-  end
-
   test "messages updated since a time are the newest page of them, in creation order" do
     room = rooms(:watercooler)
     first, second, third = room.messages.ordered.first(3)
@@ -17,15 +13,4 @@ class Message::PaginationTest < ActiveSupport::TestCase
       end
     end
   end
-
-  private
-    def plan_of_page_updated_since
-      statements = []
-      callback = ->(*, payload) { statements << payload[:sql] if payload[:sql].start_with?(%(SELECT "messages")) }
-      ActiveSupport::Notifications.subscribed(callback, "sql.active_record") do
-        rooms(:watercooler).messages.page_updated_since(1.minute.ago)
-      end
-
-      Message.connection.select_rows("EXPLAIN QUERY PLAN #{statements.sole}").map(&:last).join(" | ")
-    end
 end