Remove local maven config file and add .mvn folder to gitignore

[hmottestad]

Remove the local maven config file and add the .mvn folder to gitignore.

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PR Author Checklist (see the contributor guidelines for more details):

• [ ] my pull request is self-contained
• [ ] I've added tests for the changes I made
• [ ] I've applied code formatting (you can use mvn process-resources to format from the command line)
• [ ] I've squashed my commits where necessary
• [ ] every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change

[abrokenjester]

I'm on mobile so can't check, but I seem to remember we added this for a specific reason: the way we refer to the project root fails in Jenkins or in GitHub when this is not defined, I think. I'll try and look up the details in the commit history.

[abrokenjester]

So we introduced this file as part of the PR that introduced automated code formatting checks (see https://github.com/eclipse/rdf4j/pull/1340 ) . At the time this was done because Jenkins was having trouble with the relative path reference we use to link to the code formatting file. I can't remember if adding this was actually the fix, or if this is merely something I used to figure out the version of maven used in Jenkins.

In any case: if you want to remove it, I'd recommend running a test build on Jenkins to make sure the code formatter part of the build doesn't break there.