btrfs: collapse unaccount_log_buffer() into clean_log_buffer()

There's one only one caller of unaccount_log_buffer() and both this
function and the caller are short, so move its code into the caller.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>