discover/boot: Fix stale boot cancellation code

discover/boot: Fix stale boot cancellation code

In dc85de97 "Allow load_async_url() to call callback for local paths"
several load_url_result fields of the boot_task struct were deprecated
but were accidentally left in the struct. This caused the now out of
date code in cleanup_cancellations() to go unnoticed since it can return
safely if these fields are NULL. However freeing the boot task can free
the memory associated with each load before it is complete, resulting in
a confusing segfault.

This brings cleanup_cancellations() up to date and along the way
implicitly includes the signature resources in cleanup which were missed
originally.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>