feat(isthmus): always emit Cross relation when possible

[vbarua]

BREAKING CHANGE: removed CrossJoinPolicy from FeatureBoard

[vbarua]

The default policy is already to emit Cross relations

substrait-java/isthmus/src/main/java/io/substrait/isthmus/FeatureBoard.java

Lines 36 to 42 in 3852640

	/**
	* @return the selected cross join policy
	*/
	@Value.Default
	public CrossJoinPolicy crossJoinPolicy() {
	return CrossJoinPolicy.KEEP_AS_CROSS_JOIN;
	}

If a downstream consumer can handle the Join relation, but not the Cross relation, it should be updated the Cross relation rather than having an optional processing override in our system.