remove Scheduler

This removes the Scheduler. @JeffBezanson you've talked about doing this for awhile since it should improve performance by avoiding the extra task switch.

@westleyargentum This change set frees the sysimg build of the need for starting the Scheduler. It was needed to permit pulling in other modules into the sysimg. After this change, I can add the following lines to the bottom of my `sysimg.jl` file to have Gtk fully & instantly available at the REPL.

```julia
Base.push!(Base.DL_LOAD_PATH, "/opt/local/lib")
Base.require("Gtk")
Base.pop!(Base.DL_LOAD_PATH)
```

edit: This is WIP because I need to create a `flush_gc_msgs` Task to avoid the warning that the scheduler is being called recursively. Also need Jeff's thoughts on how `source_path` should behave (cf.  f9dd0bb).