Log subscriber API is not type-safe. File and flight recorder
subscribers are created with functions that return
weston_log_subscriber objects. But there's a problem: to destroy
these objects you have to call the right function for each type
of subscriber, and a user calling the wrong destroy function
wouldn't get a warning.

Merge functions that destroy different types of subscribers, making
the log subscriber API type-safe.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>