RFR: 8327995: Remove unused Unused_Variable

[Magnus Ihse Bursie]

This code in `globalDefinitions.hpp` is unused and should be removed:


//----------------------------------------------------------------------------------------------------
// Utility macros for compilers
// used to silence compiler warnings

#define Unused_Variable(var) var

-------------

Commit messages:
 - 8327995: Remove unused Unused_Variable

Changes: https://git.openjdk.org/jdk/pull/18242/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18242&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327995
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18242.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18242/head:pull/18242

PR: https://git.openjdk.org/jdk/pull/18242