remove declaration of JAVA17 in forge and server jobs

Discovered tonight that some jobs were setting JAVA17 = java1.8.0_last, when the correct value should have been java1.7.0_last.

But on most slaves this variable should be set automatically already; this is probably a relic from a simpler time (when JDK 7 was first being rolled out across the slaves).

So... for the _master jobs, I've removed the parameter. And for the _4.3.mars jobs, I've fixed the parameter.

If both forge and server are still blue in master after this change, we can push the same change into the _4.3.mars jobs AFTER Beta1.

https://github.com/jbdevstudio/jbdevstudio-ci/commit/6c49750510357e2959c56b1340e0579d40c2bd3d