graphs: remove the disk info

src/test/system-test.js

@@ -37,9 +37,8 @@ describe('System', function () {
         expect(memory.swap).to.be.a('number');
     });
 
-    it('can get du', async function () {
-        const usage = await system.du(__dirname, {});
-        expect(usage).to.be.a('number');
+    it('can get diskUsage', async function () {
+        const usage = await system.getDiskUsage();
+        expect(usage).to.be(null); // nothing cached
     });
 });
-