Remove Scanning::COORDINATOR_ONLY_STW

[wks]

We introduced the constant Scanning::COORDINATOR_ONLY_STW so that VMs can request the MMTk core to use a dedicated thread (the coordinator thread) to stop mutators and resume mutators. The OpenJDK binding used to require that feature, but it found a different solution, therefore Scanning::COORDINATOR_ONLY_STW is no longer necessary.

After this PR: #794, we removed the concept of "coordinator-only work" completely, and that constant Scanning::COORDINATOR_ONLY_STW became useless. We left it there as "deprecated" to give bindings a grace period to remove their uses. When no bindings are using that constant, we can remove that from MMTk-core.

Checklist:

• mmtk-openjdk (Remove deprecated const. mmtk-openjdk#217)
• mmtk-jikesrvm (The JikesRVM binding does not set the constant).
• mmtk-v8 (Note: we cannot build with V8 right now.)
• mmtk-ruby (Done in 30f95ff4d5022b15625e6f4efeeda5374ff672b2)
• mmtk-julia (The Julia binding does not set the constant).

[ArberSephirotheca]

I would like to work on it. Could you please assign me :)

[wks]

I would like to work on it. Could you please assign me :)

No need to be that formal. Just try making a PR.