posix: Deprecate custom removeAll/mkdirAll implementations. (#4808)

Since go1.8 os.RemoveAll and os.MkdirAll both support long
path names i.e UNC path on windows. The code we are carrying
was directly borrowed from `pkg/os` package and doesn't need
to be in our repo anymore. As a side affect this also
addresses our codecoverage issue.

Refer #4658

cmd/object-api-getobject_test.go

@@ -342,7 +342,7 @@ func testGetObjectDiskNotFound(obj ObjectLayer, instanceType string, disks []str
 
 	// Take 8 disks down before GetObject is called, one more we loose quorum on 16 disk node.
 	for _, disk := range disks[:8] {
-		removeAll(disk)
+		os.RemoveAll(disk)
 	}
 
 	// set of empty buffers used to fill GetObject data.