Revert "Run single step as single-node job graph via scheduler"
Revert "Run single step as single-node job graph via scheduler"

Prefer extracting single-step run method, and calling it from scheduler
at each step. Seems a simpler implementation, rather than threading
this run-single-step behaviour top-down through the scheduler.

This reverts commit 4d92ac599fe9584dc7c722ba93915b027b1dd6f6.

nismod/smif,nismod/smif,nismod/smif,tomalrussell/smif,tomalrussell/smif,tomalrussell/smif,nismod/smif,tomalrussell/smif