RFR: 8279522: Serial: Remove unused Generation::clear_remembered_set

[Albert Mingkun Yang]

Trivial change of removing unused code.

Test: hotspot_gc

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/6967/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6967&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279522
  Stats: 12 lines in 3 files changed: 0 ins; 12 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6967.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6967/head:pull/6967

PR: https://git.openjdk.java.net/jdk/pull/6967