RFR: 8273062: Generation::refs_discovery_is_xxx functions are unused

[Kim Barrett]

Please review this trivial cleanup, removing a couple of unused virtual
function declarations from the Generation class:

  virtual bool refs_discovery_is_atomic() const { return true;  }
  virtual bool refs_discovery_is_mt()     const { return false; }

These functions are not called or overridden.

Testing:
local (linux-x64) build.

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Commit messages:
 - remove unused declarations

Changes: https://git.openjdk.java.net/jdk/pull/5275/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5275&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273062
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5275.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5275/head:pull/5275

PR: https://git.openjdk.java.net/jdk/pull/5275