revert minor changes matrix block [skip ci]

Author: Baunsgaard

src/main/java/org/apache/sysds/runtime/matrix/data/MatrixBlock.java

@@ -1424,8 +1424,6 @@ public long recomputeNonZeros(int k) {
 		if(k <= 1 || ((long) rlen * clen < 10000) || (sparse && sparseBlock!=null))
 			return recomputeNonZeros();
 		else if(!sparse && denseBlock!=null){
-			if((long) rlen * clen < 10000 || k == 1)
-				return recomputeNonZeros();
 			final ExecutorService pool = CommonThreadPool.get(k);
 			try {
 				List<Future<Long>> f = new ArrayList<>();
@@ -1453,10 +1451,6 @@ else if(!sparse && denseBlock!=null){
 					nnz += e.get();
 				nonZeros = nnz;
 
-				if(nonZeros < 0)
-					throw new DMLRuntimeException("Invalid count of non zero values: " + nonZeros);
-				return nonZeros;
-
 			}
 			catch(Exception e) {
 				LOG.warn("Failed Parallel non zero count fallback to single thread");