[REM] mail, hr: remove old redirection when posting on users

Posting on users is not supported anymore. It was implemented in a time
we thought having a twitter-like behavior in Odoo was a good idea.
Posting on users was redirected on its related partner in mail. It was
redirected on its related employee in hr. It does not make sense anymore
as employee are records with limited access and partners used for other
purpose than discussing notably in accounting.

Now that we have chat channels it makes no sense to have this kind of
behavior still implemented. Let us remove it. No functional change
should occur with this commit.