Clean up findbugs-exclude.xml files

There are some "empty" exclude-filter.xml files which exist solely so findbugs-maven-plugin doesn't complain that it's absent.

These can be cleaned up using a profile to only activate the excludes filtering when the file exists.

Additionally, there's a vestigial findbugs exclude file in the contrib directory which, as far as I can tell, serves no purpose at all and should be removed.