devel/flexdock: unpin openjdk8

Builds fine with modern JDK.
Dependency math/scilab uses openjdk8 so this needs to generate
Java 8 compatible class files.

NB: I think upstream moved here: https://gitlab.com/scilab/forge/flexdock

NB2: Remove java:run as it is a library. The final application will decide
what JDK to run with, so no need to install multiple JDKs.

Approved-by: no maintainer