Remove content from .gitignore

IDE files should be ignored by the developers not the project (set that
in the global git ignore, or in the local repo settings).
Old generated files do not need to be ignore anymore.
Only files generated during the build should be ignored (so only target/)
Avoid using Raven-Java when possible

Java is trademarked, it's fine to know that the project is maven for Java
but Raven-Java could become a problem