[FIX] mail: followers: can't access res.partner case

When the user can't read the model res.partner, former code tried to determine
whether or not the user was follower of the document. This was wrong for three
reasons.

First, it performed an RPC to get the partner_id of the current user, but this
info is already stored in the session so the RPC was useless.
Second, the partner_id isn't helpful to know if the user is a follower because
everything we have is the follower_ids, which aren't ids of the res_partner
model, but ids of the mail_follower model (compare apples and oranges...).
Third, we anyway didn't use that information at all as the button indicating
whether or not the user is a follower wasn't displayed.

This rev. simply removes that useless RPC and only displays the count of
followers as previously.