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/storage-rpc-server_test.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"os"
 	"testing"
 
 	"github.com/minio/minio/pkg/disk"
@@ -78,7 +79,7 @@ func errorIfInvalidToken(t *testing.T, err error) {
 func TestStorageRPCInvalidToken(t *testing.T) {
 	st := createTestStorageServer(t)
 	defer removeRoots(st.diskDirs)
-	defer removeAll(st.configDir)
+	defer os.RemoveAll(st.configDir)
 
 	storageRPC := st.stServer