[FIX] website_event_track: rename Partner to Speaker and remove duplicated fields

Before this patch, the word "Partner" wasn't specific enough to make the user
know that it was referring to the speaker. This was OK in the form, bug i.e.
in the calendar view it's very confusing.

This change comes from 1e61522d and we revert back to Speaker labelling.

Also, in the form view, the partner field was duplicated, contributing even
more to the confusion. It probably comes from c058562d that duplicated the field
probably because of a bad conflict resolution.

Now partner is labelled Speaker and multi company domain is correctly taken
into account.

closes odoo/odoo#37781

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>