RFR: 8303067: G1: Remove unimplemented G1FullGCScope::heap_transition

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/12712/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12712&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303067
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12712.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12712/head:pull/12712

PR: https://git.openjdk.org/jdk/pull/12712