RFR: 8339346: GenShen: Remove even more stale TODOs

[William Kemper]

Some of these have been done, some of these will not be done, some have been moved into our ticketing system, some of them are done in this PR.

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Commit messages:
 - oop arguments do not need to be const
 - Checkpoint
 - Rename `object_starts` to `_object_starts`
 - Remove unused methods and fields
 - Merge remote-tracking branch 'shenandoah/master' into fix-stale-todos
 - Implement TODO to use an API for excluding free regions
 - Remove or rephrase confusing TODO items
 - Remove out-dated TODOs
 - Remove invalid TODO and vestige of IU mode
 - No, we must use a mask to test for a set bit
 - ... and 2 more: https://git.openjdk.org/shenandoah/compare/c7cfc7fa...c3a0301c

Changes: https://git.openjdk.org/shenandoah/pull/491/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339346
  Stats: 205 lines in 18 files changed: 0 ins; 176 del; 29 mod
  Patch: https://git.openjdk.org/shenandoah/pull/491.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/491/head:pull/491

PR: https://git.openjdk.org/shenandoah/pull/491