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/lock-rpc-server-common_test.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"os"
 	"reflect"
 	"testing"
 	"time"
@@ -25,7 +26,7 @@ import (
 // Test function to remove lock entries from map only in case they still exist based on name & uid combination
 func TestLockRpcServerRemoveEntryIfExists(t *testing.T) {
 	testPath, locker, _ := createLockTestServer(t)
-	defer removeAll(testPath)
+	defer os.RemoveAll(testPath)
 
 	lri := lockRequesterInfo{
 		writer:          false,
@@ -62,7 +63,7 @@ func TestLockRpcServerRemoveEntryIfExists(t *testing.T) {
 // Test function to remove lock entries from map based on name & uid combination
 func TestLockRpcServerRemoveEntry(t *testing.T) {
 	testPath, locker, _ := createLockTestServer(t)
-	defer removeAll(testPath)
+	defer os.RemoveAll(testPath)
 
 	lockRequesterInfo1 := lockRequesterInfo{
 		writer:          true,