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-list-objects-heal_test.go

@@ -18,6 +18,7 @@ package cmd
 
 import (
 	"bytes"
+	"os"
 	"path"
 	"path/filepath"
 	"strconv"
@@ -34,7 +35,7 @@ func TestListObjectsHeal(t *testing.T) {
 		t.Fatalf("Init Test config failed")
 	}
 	// remove the root directory after the test ends.
-	defer removeAll(rootPath)
+	defer os.RemoveAll(rootPath)
 
 	// Create an instance of xl backend
 	xl, fsDirs, err := prepareXL()
@@ -151,7 +152,7 @@ func TestListUploadsHeal(t *testing.T) {
 		t.Fatalf("Init Test config failed")
 	}
 	// Remove config directory after the test ends.
-	defer removeAll(rootPath)
+	defer os.RemoveAll(rootPath)
 
 	// Create an instance of XL backend.
 	xl, fsDirs, err := prepareXL()