luci-app-attendedsysupgrade: remove outdated checks

luci-app-attendedsysupgrade: remove outdated checks

the upgrade server used to store a md5 checksum and the image filesize,
due to complexity this was removed.

Instead a sha256sums file is created by the IB with additional
signature. The firmware filesize can be retrived by requesting
Content-Length data.sysupgrade_url.

As the deeper layers (rpcd) currently do not offer an easy way to to
perform a sha256sum check & verify a usign sig these features will be
re-added once supported.

Until then the security of the sysupgrades depend on TLS and so the
luci-app is meant as a demonstration only.

Signed-off-by: Paul Spooren <paul@spooren.de>