How to reproduce:

1. Open attached project
- Notice the warning upon compilation
2. Add a 'new' keyword after the 'private' keyword in the script
- The editor gives no warnings
3. Build to any platform
- Notice Unity gives a warning that 'new' is not necessary

For a simple workaround, please surround the offending code like this:

#pragma warning disable 0108
private Rigidbody rigidbody = null;
#pragma warning restore 0108

This disables (and then reenables) the Editor warning that says that 'rigidbody' is being hidden.