PURPOSE
Simplify 'mail.tracking.value' model and code. Remove unnecessary fields and
computation. Make code easier to handle and more batch-enabled.
SPECIFICATIONS
In old times, 'account' and 'project' supported 2many fields tracking. Then
it was moved directly into 'mail'. Thanks to precommit hooks and complete
record access for relational fields, it is now possible to track 2many
fields.
In this commit we cleanup odoo/odoo@167944cc93 that landed during work on this task and
add some tests to be sure it is effectively covered.
Task-3345979 (Mail: Simplify tracking model)
Part-of: odoo/odoo#124182
Thibault Delavallée
6344fc7d50