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/xl-v1-object_test.go

@@ -269,7 +269,7 @@ func TestHealing(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to initialize test config %v", err)
 	}
-	defer removeAll(rootPath)
+	defer os.RemoveAll(rootPath)
 
 	obj, fsDirs, err := prepareXL()
 	if err != nil {