Bug#959930: update does not need gradle

[Andrius Merkys]

control: unblock -1 by 933264 926714

I have successfully built v0.39 locally without gradle (using javahelper
instead). Maven coordinates have disappeared as a result, but at least
one of the reverse dependencies, beast-mcmc, builds successfully. Other
reverse dependencies (deepboof, ddogleg and georegression) FTBFS because
of the missing Maven coordinates, but this can be patched or resolved by
switching these packages away from gradle to javahelper too.

I plan to push libejml-java update soon and turn to the failing reverse
dependencies (low popcon, maintained by me).

Andrius