RFR: JDK-8299971: Remove metaprogramming/conditional.hpp

[Justin King]

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

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Commit messages:
 - Remove metaprogramming/conditional.hpp

Changes: https://git.openjdk.org/jdk/pull/11946/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11946&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299971
  Stats: 136 lines in 11 files changed: 7 ins; 105 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/11946.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11946/head:pull/11946

PR: https://git.openjdk.org/jdk/pull/11946