[REF] mail: simplify email send code and remove unnecessary methods

Send methods of mail.mail model are set private and simplified. Those
methods have been written when a lot of overrides were necessary to
generate notification emails. Since then the flow has changed but
methods have not been updated completely. This commit therefore simplified
a bit the call chain and parameters.

This commit is purely technical and does not hold any functional change.
Purpose is to clean code in order to prepare future changes in mail
application.

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