Remove Scheduler, SchedulerOptions (#5188)

Summary:
Pull Request resolved: #5188

**Context**: This was deprecated in Ax 1.0.0, so it is overdue for reaping.

**Changes**: Remove `Scheduler`, `SchedulerOptions`, and `SchedulerInternalError`

Reviewed By: mpolson64

Differential Revision: D101829778

fbshipit-source-id: 86afd3654f05e81daeeab76bcffac17eb88ea10e

Author: esantorella