RFR: Unconditional conditional card marking

[Aleksey Shipilev]

For the overwhelming majority of current systems, it makes little sense to run without conditional card marks enabled. G1, for example, makes its card marks unconditional. In other words, G1 does not respond to `UseCondCardMark`. This also simplifies code, eliminates one additional testing configuration, and provides safety for the cases where `UseCondCardMark` is accidentally disabled.

Additional testing: 
 - [x] macos-aarch64-server-fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Unconditional conditional card marking

Changes: https://git.openjdk.org/shenandoah/pull/264/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=264&range=00
  Stats: 40 lines in 4 files changed: 4 ins; 22 del; 14 mod
  Patch: https://git.openjdk.org/shenandoah/pull/264.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/264/head:pull/264

PR: https://git.openjdk.org/shenandoah/pull/264