[IMP] event(_sale): perform some code removal / linting

PURPOSE

As event will soon evolve (onchange -> compute, code improvements, addition of
new features) cleaning and improving tests is necessary to help avoid issues.

SPECIFICATIONS

Use _tz_get for tz-based selection fields from partner as it smartly order
available timezones.

Remove a strange "event.confirm" wizard that calls an unexisting method,
is not reachable and is actually completely unnecessary.

Reorganize ticket fields to ease future improvements and perform some
public -> private method cleaning.

Remove duplicated code in event_sale.

LINKS

Side effect of Task ID 2089156 (event onchange to compute)
Community PR odoo/odoo#43127
Enterprise PR odoo/enterprise#7656

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