RFR: 8301768: G1: Remove unimplemented HeapRegionRemSet::split_card

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/12408/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12408&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301768
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12408.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12408/head:pull/12408

PR: https://git.openjdk.org/jdk/pull/12408