Disable maven-site-plugin reports that don't add any value

[marcphilipp]

• Ref -- simply browse the code on GitHub
• changes -- was empty anyways
• GitHub issues -- outdated after ten minutes, simply browse the issues on GitHub
• Surefire -- not useful to look at test results that are days, weeks, or months old
• Maven plugins -- take a look at the POM
• Summary -- says nothing new

[marcphilipp]

Before we merge, I can squash the commits of course. However, I would prefer not to, so one can reconstruct what commit disabled which report.

[stefanbirkner]

Thanks.