Remove settings-example.xml and any reference to it

Following HSEARCH-2988, the custom repository settings are hard-coded into the only POMs that require it (those that use a WildFly server).
Thus we don't need settings-example.xml. Let's remove it along with any mention of it:

• .travis.yml

• README.md

• CI jobs (we currently use -s settings-example.xml)

• "Contributing" guide, if any?

• HTML documentation?

• hibernate.org, at least for 5.9/5.10?