RFR: JDK-8300265: Remove metaprogramming/isSigned.hpp

[Justin King]

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

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Commit messages:
 - Remove metaprogramming/isSigned.hpp

Changes: https://git.openjdk.org/jdk/pull/12042/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12042&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300265
  Stats: 105 lines in 6 files changed: 3 ins; 91 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12042.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12042/head:pull/12042

PR: https://git.openjdk.org/jdk/pull/12042