API/DEPR: Remove +/- as setops for DatetimeIndex/PeriodIndex (GH9630)
xref #13777, deprecations put in place in #9630

Some todo's:
- still have to further clean-up the tests a bit (due to the previous API the tests for set ops and numeric ops are a bit mixed)
- need to add some more test cases for DTI - DTI (eg with NaNs)
- for subtraction of two `PeriodIndex` objects, this is currently raising a TypeError, but I think we could actually allow it? Although that maybe depends on what is decided in https://github.com/pydata/pandas/issues/13077

I think these should be the last remaining set ops as numeric ops that are now removed with this PR.
 we need to wait for #13077 on `Period-Period` (so can discuss over there). Let's limit scope to the more useful DTI-DTI
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14164?src=pr) is 85.25% (diff: 96.55%)

> Merging [#14164](https://codecov.io/gh/pydata/pandas/pull/14164?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14164   diff @@
==========================================
  Files           139        139          
  Lines         50496      50512    +16   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43049      43064    +15   
- Misses         7447       7448     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8023029...21c6cec](https://codecov.io/gh/pydata/pandas/compare/80230292e19e377f5b11409e112500b6a180e18a...21c6cec62ae2ad1b3aa8356962365fdf0383c5d2?src=pr)
 lgtm.
