RFR: 8306833: Change CardTable::_covered to static array [v4]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13636/files
  - new: https://git.openjdk.org/jdk/pull/13636/files/760fbe3b..63ce68bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13636&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13636&range=02-03

  Stats: 6 lines in 2 files changed: 2 ins; 4 del; 0 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