RFR: 8261430: Remove redundant #ifndef HEADLESS checks from metal native code

[Ajit Ghaisas]

Files in macosx/native/libawt_lwawt/java2d/metal directory have redundant #ifndef HEADLESS checks which are removed.
 
This was identified as part of code review in https://github.com/openjdk/jdk/pull/2403

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Commit messages:
 - 8261430 - remove HEADLESS check

Changes: https://git.openjdk.java.net/lanai/pull/177/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=177&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261430
  Stats: 40 lines in 10 files changed: 0 ins; 40 del; 0 mod
  Patch: https://git.openjdk.java.net/lanai/pull/177.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/177/head:pull/177

PR: https://git.openjdk.java.net/lanai/pull/177