Remove unused factor levels from data after limiting

After limiting the data there may be factors with empty levels. This can
be confusing and should be avoided.

utils.R

@@ -77,6 +77,9 @@ sma_load_data <- function(file) {
 	# At this time only solid tumors are important
 	sma <- filter(sma, diagnosis_type == "solid")
 
+	# After filtering unused levels must be cleared from the data
+	sma <- droplevels(sma)
+
 	return(sma)
 }