community/dcron: don't specify custom SCRONTABS

Instead use the default value /etc/cron.d. The /etc/periodic directory
(previously used for SCRONTABS) contains subdirectories with cronjobs
for specific intervals, installed by the Alpine packaging system. dcron
doesn't understand these subdirectories. Furthermore, cronjobs in these
directories are invoked using run-parts from /etc/crontabs/root
as provided by alpine-baselayout.

Users can now manually install crons to /etc/cron.d if desired.