RFR: 8278251: Enable "missing-explicit-ctor" check in the jdk.unsupported.desktop module

[Sergey Bylokhov]

The "missing-explicit-ctor" check was disabled by the [JDK-8071961](https://bugs.openjdk.java.net/browse/JDK-8071961) and later was fixed by the [JDK-8250853](https://bugs.openjdk.java.net/browse/JDK-8250853). So we can re-enable this check again.

The fix will remove the "Java.gmk" file and as a result the "jdk.unsupported.desktop" folder.

All "Pre-submit tests" builds are green.

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Commit messages:
 - Delete Java.gmk

Changes: https://git.openjdk.java.net/jdk/pull/6708/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6708&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278251
  Stats: 26 lines in 1 file changed: 0 ins; 26 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6708.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6708/head:pull/6708

PR: https://git.openjdk.java.net/jdk/pull/6708