agenda: Cut out huge swath of old 8sync-* procedures/macros.

* 8sync/agenda.scm (<wrapped-exception, make-wrapped-exception)
  (wrapped-exception?, wrapped-exception-key, wrapped-exception-args)
  (wrapped-exception-stacks, propagate-%async-exceptions, 8sync-run)
  (8sync-run-at, 8sync-run-delay, 8sync-delay): Removed.
  (8sync): Update to allow for multiple statements in body.
  (8sleep): Rewrite to not depend on 8sync-delay.
* tests/test-agenda.scm: Remove test for 8sync-delay.