weston-log/weston-log-wayland: Inline private subscription functions

This avoids duplicated bits, by calling the scopes's callback (if any)
and adding the subscription to the scope's subscription list. Further
more, the scope's name when creating the subscription is not needed so
removed that as well.

In mirror, also inline removing of subscription for scope's subscription
list. Fix a potential corner case when the user can request a
subscription to an invalid scope in stream_destroy().

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