Introduced with e0a858a5, commit 'weston-debug: Introduce
weston_log_subscription and weston_log_subscriber objects'. We don't
really return a weston_log_subscription so let's remove it.

Some newer doxygen detects this and we are treating warning as errors.

Fixes #594

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>