Returns only the relevant combinations from a list of word combinations. Assumes
occurrences have already been calculated.

@param {WordCombination[]} wordCombinations A list of word combinations.
@returns {WordCombination[]} Only relevant word combinations.
function getRelevantCombinations( wordCombinations ) {
	wordCombinations = wordCombinations.filter( function( combination ) {
		return combination.getOccurrences() !== 1 &&
			combination.getRelevance() !== 0;
	} );
	return wordCombinations;
}