Rework handling of fully active status

[rakuco]

Related to #275, where it is explained that the current model of activating
data delivery again when a document becomes fully active is unclear.

Furthermore, doing the same thing to workers based on whether they are
active needed workers or not does not have precedent in other specs and it
is not clear if the current steps even work or not.

Instead, do the following:

• Remove the steps that handle a document becoming fully active again and
  point to Properly determine bfcache integration strategy #275.
• Remove the steps that handle a worker becoming an active needed worker
  again for symmetry (and also because it is not clear if it can become an
  active worker again once it it stops being so).
• Merge the "Handle unloading document and closing of workers" section into
  the "Handling change of fully active" one.
  • For documents, the former was basically duplicating the latter. In fact,
    the "unloading document cleanup steps" are how specs are supposed to
    react to a document no longer being fully active.
  • For workers, the "active needed worker" references have been replaced by
    a check for the value of the closing flag in WorkerGlobalScope.
• Use proper algorithm steps instead of prose to outline the steps that must
  be performed in both cases.

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