Do not include ActionView::Helpers::UrlHelper

What does this MR do and why?

This MR addresses #341403 (closed).

It removes the inclusion of ActionView::Helpers::UrlHelpers in lib/gitlab/email/message/in_product_marketing/helper.rb to avoid adding other methods from ActionView::Helpers::UrlHelpers when we only want to use link_to. Instead of including the whole module, link_to is directly called from ActionController::Base.helpers as suggested in #340567 (comment 674155273).

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