Remove external df module

It has some parsing issues with locale

src/test/system-test.js

@@ -22,6 +22,7 @@ describe('System', function () {
 
         const disks = await system.getDisks();
         expect(disks).to.be.ok();
+        expect(disks.some(d => d.mountpoint === '/')).to.be.ok();
     });
 
     it('can check for disk space', async function () {