RFR: Clean up CondCardMark code: remove excess barriers, enable it by default

[Aleksey Shipilev]

The `StoreLoad`, `StoreStore` barriers in that code are only needed for CMS, and does not relate to Shenandoah. Also, `UseCondCardMark` is a recommended option for most machines today, and therefore we better just enable it ergonomically.

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Commit messages:
 - Clean up CondCardMark code: remove excess barriers, enable it by default

Changes: https://git.openjdk.java.net/shenandoah/pull/26/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=26&range=00
  Stats: 20 lines in 4 files changed: 3 ins; 16 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/26/head:pull/26

PR: https://git.openjdk.java.net/shenandoah/pull/26