Delete solr/core/src/test-files/solr/configsets/_default

For as long as we've had a _default configset in solr, we've also had a copy of that default in core/src/test-files/ - as well as a unit test that confirms they are identical.

It's never really been clear to me why we have this duplication, instead of just having the test-framework take the necessary steps to ensure that server/solr/configsets/_default is properly used when running tests.

I'd like to propose we eliminate the duplication since it only ever seems to cause problems (notably spurious test failures when people modify the _default configset w/o remembering that they need to make identical edits to the test-files clone) and instead have SolrTestCase set the (already existing & supported) solr.default.confdir system property to point to the (already existing) ExternalPaths.DEFAULT_CONFIGSET