RFR: JDK-8299398: Remove metaprogramming/isConst.hpp

[Justin King]

Code cleanup of pre-C++11 <type_traits> implementations.

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Commit messages:
 - Remove related test
 - Remove metaprogramming/isConst.hpp

Changes: https://git.openjdk.org/jdk/pull/11800/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299398
  Stats: 84 lines in 3 files changed: 2 ins; 81 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11800.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11800/head:pull/11800

PR: https://git.openjdk.org/jdk/pull/11800