RFR: 8305618: Move gcold out of tier1

[Leo Korinth]

8305618: Move gcold out of tier1

Remove gcold out from tier1. Related to [JDK-8298981](https://bugs.openjdk.org/browse/JDK-8298981). Moving gcbasher out of tier1 was more controversial, and will be done later --- if at all.

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Commit messages:
 - removed gcold from tier1_gc

Changes: https://git.openjdk.org/jdk/pull/13340/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13340&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305618
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13340.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13340/head:pull/13340

PR: https://git.openjdk.org/jdk/pull/13340