[FIX] mail: remove useless hasattr on get_base_url

This check was introduced with d82b897e in 12.0 where `get_base_url` was
not yet a model method.

Since f727cd26, in saas-12.5, that method is now on model, making the
`hasattr` check useless.

closes odoo/odoo#35057

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>