[r=cjwatson] Drop Supported field, no longer accurate and thus obsolete
With introduction of new support subscriptions the support status and length
of packages at initial release publication are not known. We do not modify
release pocket, and thus when a release gets extended to beyond 5y for some
or all packages, the Supported field stops being accurate.
Stop publishing the confusing Supported field.
A tool that reports correct security support status will be provided soon,
which reports correct status and does not use Supported field anymore.