[REF] Remove loading contribution page id from passed in object

I believe this became obsolete with this PR
https://github.com/civicrm/civicrm-core/commit/6c3e774c322267c8e4449006db9cc14202b7028f

The other 2 (paypal) IPN classes no longer set contribution_page_id deliberately so
would be relying on the same data used for getTemplateContribution