RFR: 8255798: Remove dead headless code in CompileJavaModules.gmk

[Magnus Ihse Bursie]

The variable BUILD_HEADLESS_ONLY is no longer set. And sun/applet does not exist anymore.

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Commit messages:
 - 8255798: Remove dead headless code in CompileJavaModules.gmk

Changes: https://git.openjdk.java.net/jdk/pull/1031/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1031&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255798
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1031.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1031/head:pull/1031

PR: https://git.openjdk.java.net/jdk/pull/1031