RFR: 8240751: Shenandoah: fold ShenandoahTracer definition

[Aleksey Shipilev]

Currently, we have `shenandoahTracer.hpp` with a sole definition:
class ShenandoahTracer : public GCTracer {
public:
  ShenandoahTracer() : GCTracer(Shenandoah) {}
};

This can be folded near the only use at `shenandoahCollectorPolicy.hpp`.

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Commit messages:
 - JDK-8240751: Shenandoah: fold ShenandoahTracer definition

Changes: https://git.openjdk.java.net/jdk/pull/55/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=55&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240751
  Stats: 41 lines in 2 files changed: 6 ins; 35 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/55.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/55/head:pull/55

PR: https://git.openjdk.java.net/jdk/pull/55