[FIX] mail: remove duplicate code

There already is a redirect implementation in the chat_manager
which closely matches the on_redirect implementation in chatter.js,
however the former has the advantage of calling get_formview_id
to ensure that the view is correct.

This fixes a faulty behaviour when some models are referenced from
chatter message using data attribute data-oe-model and data-oe-id.
For example, linking invoices in the chatter would always open them
with the 'vendor invoice' view; from this revision on the correct
view will be used (assuming get_formview_id is implemented on the
model).

The default behaviour of the chat_manager when a user clicks on a
partner record is to open a channel if the partner has a user then
return a callback passed as a parameter (usually to open the channel
as a popup or in the discuss app). To keep the old behaviour
of the chatter followers list widget (i.e. open the partner form
when clicking on a follower), a small change has to be made on the
redirect implementation: when no callback is provided, do not create
the channel (instead a returning an empty callback).