RFR: 8265867: thread.hpp defines some enums but no reference

[Xin Liu]

Deleted 2 useless enums.
This patch also added const and constexpr for the applicable member functions.

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Commit messages:
 - 8265867: thread.hpp defines some enums but no reference

Changes: https://git.openjdk.java.net/jdk/pull/3702/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3702&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265867
  Stats: 104 lines in 4 files changed: 0 ins; 12 del; 92 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3702.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3702/head:pull/3702

PR: https://git.openjdk.java.net/jdk/pull/3702