Remove NuxeoPrincipal instanciation when building user workspace

When building a user workspace, a NuxeoPrincipal is instanciated to get the firstname and lastname and set user workspace title.

Actually building a NuxeoPrincipal for that is not necessary and using user model from UserManager is enough.
Moreover, not building a NuxeoPrincipal at this step will prevent from getting StackOverflowException, when having computed groups that need information from user profile