agenda: Remove 8sync macro.

Despite being the namesake of the project, this turns out to have never
really been important or made sense, except as a way to yield control as
a form of context switching.  This is moving towards renaming
8sync-nowait to 8sync, a forthcoming change.

* 8sync/agenda.scm (8sync, catch-8sync): Removed.
* tests/test-agenda.scm: Removed all tests calling the 8sync macro.