services: Make exim-service-type use mail-aliases-service-type

* gnu/services/mail.scm (exim-configuration)[aliases]: Remove field.
(exim-activation, exim-shepherd-service): Remove alias from matches.
(exim-etc): Remove procedure.
(exim-service-type): Extend mail-aliases-service-type instead of
etc-service-type.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>