RFR: 8306749: Make CardTable::invalidate non-virtual

[Albert Mingkun Yang]

Trivial removing unused `virtual` specifier.

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Commit messages:
 - card-table-remove-virtual

Changes: https://git.openjdk.org/jdk/pull/13617/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13617&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306749
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13617.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13617/head:pull/13617

PR: https://git.openjdk.org/jdk/pull/13617