REF: change TDI.delete behavior to match DTI.delete
With this change, the two methods behave the same and can be shared.
Whats the behavior change between "infer" and None? > Whats the behavior change between "infer" and None?

```
tdi = pd.TimedeltaIndex(["1 Day", "2 Days", None, "3 Days", "4 Days"])

>>> tdi.delete(2).freq
None   <-- PR
<Day>  <-- master
```
 ping