RFR: 8306833: Change CardTable::_covered to static array

[Albert Mingkun Yang]

Simple refactoring to remove unused declaration in `vmStructs_gc.hpp`, which then enables more cleanup in `CardTable`.

Test: hotspot_gc

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Commit messages:
 - card-table-array-field

Changes: https://git.openjdk.org/jdk/pull/13636/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13636&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306833
  Stats: 10 lines in 3 files changed: 0 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13636.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13636/head:pull/13636

PR: https://git.openjdk.org/jdk/pull/13636