Remove `ufUniqID` from Core

I'm auditing the synchronize() procedure with the aim of rationalising what happens during page loads in WordPress - due to reports of multiple Contacts being created (see this Mattermost thread for example) and have found that the ufUniqID session variable is only ever set and never read in CiviCRM.

It is set in Core in:

CRM_Core_BAO_UFMatch::synchronize() here and here

And in CiviCRM WordPress by the REST API (presumably for consistency with Core) in:

CiviCRM_WP_REST\Civi\Mailing_Hooks\Plugin::set_civi_user_session() here

Can we remove it since it's never actually used?

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