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/fs-v1-rwpool_test.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"os"
 	"runtime"
 	"testing"
 
@@ -50,7 +51,7 @@ func TestRWPool(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to create posix test setup, %s", err)
 	}
-	defer removeAll(path)
+	defer os.RemoveAll(path)
 
 	rwPool := &fsIOPool{
 		readersMap: make(map[string]*lock.RLockedFile),