RFR: 8307005: Make CardTableBarrierSet::initialize non-virtual

[Albert Mingkun Yang]

Trivial removing `virtual` specifier.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/13713/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13713&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307005
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13713.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13713/head:pull/13713

PR: https://git.openjdk.org/jdk/pull/13713