Revert "[Build] Set Java Compiler's -release option instead of -source + -target"

[NiklasRentzCAU]

Reverts #214

Seems like the PR #214 did lead to errors not occuring in the PR build due to Javadoc building:

Error:  Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.5.0:jar (attach-javadocs) on project de.cau.cs.kieler.klighd.kgraph: MavenReportException: Error while generating Javadoc: 
Error:  Exit code: 1
Error: [ERROR] error: module org.eclipse.emf.ecore.xmi reads package org.eclipse.core.runtime from both org.eclipse.equinox.common and org.eclipse.equinox.registry
Error: [ERROR] error: module org.eclipse.emf.ecore.xmi reads package org.eclipse.core.internal.runtime from both org.eclipse.equinox.common and org.eclipse.core.runtime
Error: [ERROR] error: module org.eclipse.emf.ecore.xmi reads package org.eclipse.core.runtime from both org.eclipse.equinox.common and org.eclipse.core.runtime
Error: [ERROR] error: module org.eclipse.elk.graph reads package org.eclipse.core.runtime from both org.eclipse.equinox.common and org.eclipse.equinox.registry
Error: [ERROR] error: module org.eclipse.elk.graph reads package org.eclipse.core.internal.runtime from both org.eclipse.equinox.common and org.eclipse.core.runtime
[...]

Reverting this PR.