Increase spotbugs checks and fix redundant interface declaration

[aneveux]

Hello folks :wave:

I'm currently following the Improve a Plugin documentation for Jenkins plugins modernization, and noticed we could add more spotbugs checks for this plugin.

By doing so and running mvn -DskipTests verify, Spotbugs reported that com.cloudbees.jenkins.plugins.awscredentials.AWSCredentialsImpl was implementing the interface AmazonWebServicesCredentials, but so is BaseAmazonWebServicesCredentials, which AWSCredentialsImpl extends, so I removed that redundant interface declaration.

I checked that everything was still fine with mvn verify and everything looks good.

Let me know if I missed anything with this PR.

Thanks a lot for your review!

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[aneveux]

Hello @jglick , @Vlatombe ,

Can I get a quick review on that one please? :heart: