RFR: 8293164: Remove unimplemented Generation::print_heap_change

[Albert Mingkun Yang]

Trivial change of removing dead code.

Test: build

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/10098/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10098&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293164
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10098.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10098/head:pull/10098

PR: https://git.openjdk.org/jdk/pull/10098