RFR: 8253928: G1: Remove G1ConcurrentMarkThread::set_in_progress declaration

[Thomas Schatzl]

There is no definition of G1ConcurrentMarkThread::set_in_progress since JDK-8240556 any more, so also remove the
declaration in this trivial change.

Testing: local compilation

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/480/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=480&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253928
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/480.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/480/head:pull/480

PR: https://git.openjdk.java.net/jdk/pull/480