RFR: 8306833: Change CardTable::_covered to static array [v2]

[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 with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - protected-destructor
 - card-table-array-field

-------------

Changes: https://git.openjdk.org/jdk/pull/13636/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13636&range=01
  Stats: 24 lines in 6 files changed: 7 ins; 12 del; 5 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