mod_s2s_log: Remove ?DEBUG() calls

Current ejabberd versions support two different logging frameworks that
require different build options.  There's currently no easy way to deal
with this for external modules, so let's omit the ?DEBUG() calls for the
moment.
mod_s2s_log: Update gen_mod:get_opt() call

Adjust the gen_mod:get_opt() call to work with current ejabberd
versions.
Merge pull request #27 from weiss/fix-s2s-log

Fix mod_s2s_log to work with ejabberd 13.12