RFR: JDK-8299479: Remove metaprogramming/decay.hpp

[Justin King]

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

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Commit messages:
 - Remove header
 - Remove include
 - Remove metaprogramming/decay.hpp

Changes: https://git.openjdk.org/jdk/pull/11816/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11816&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299479
  Stats: 103 lines in 3 files changed: 0 ins; 89 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/11816.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11816/head:pull/11816

PR: https://git.openjdk.org/jdk/pull/11816