Remove original_string accessor for Value::Number

Forgot to remove this in 4d5a397. In the process, discovered that
`ZeroUnit` was still referencing `original_string` even though it didn't
need to.

Change-Id: Ic55ddb1bc22e3be852ce38cac7dc1db521db0f45
Reviewed-on: http://gerrit.causes.com/36428
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>