package/dovecot: bump version to 2.2.18

Removed patch applied upstream:
http://hg.dovecot.org/dovecot-2.2/rev/e4ad83ed88c9



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>