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

[Joe Darcy]

On Fri, 3 Dec 2021 20:55:23 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

> 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.

Not a reviewer for this particular change, but I support its goals.

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