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/prepare-storage-msg_test.go

@@ -18,6 +18,7 @@ package cmd
 
 import (
 	"fmt"
+	"os"
 	"testing"
 )
 
@@ -27,7 +28,7 @@ func TestHealMsg(t *testing.T) {
 	if err != nil {
 		t.Fatal("Unable to initialize test config", err)
 	}
-	defer removeAll(rootPath)
+	defer os.RemoveAll(rootPath)
 	storageDisks, fsDirs := prepareXLStorageDisks(t)
 	errs := make([]error, len(storageDisks))
 	defer removeRoots(fsDirs)