[REF] mail: simplify parent fetch when posting a message

1d57a091 added a two-steps parent fetch when posting a message
on a discussion thread. At the time this commit was done responsible were
notified of the first thread message. It was helpful to notify them of the
original email that created the thread instead of the "document created"
generic message. Indeed for documents like leads or issues having the
original email containing the customer question is interesting when being
notified. This is why this commit was done.

However since quite a long time responsible notifications have changed.
They now receive an email notification telling they have been assigned on
a document. This commit is not necessary anymore and we therefore remove
it in order to clean a bit the message_post process.

This commit globally reverts 1d57a091. Please note that @beledouxdenis
agreed after a long thinking.

This commit has a slight impact on mail performance tests, allowing to save
one query in some more complex test case.