Move Unchanged arith methods to EA mixins
Next steps after #19902.  Breaking these up into smaller pieces than usual because I don't know what bits will prove controversial.

All this PR does is take a few of the easiest no-changes-needed methods and moved them to the EA mixins.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=h1) Report
> Merging [#21712](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7cd2679982138e1e7f9f1a759827647e6e521d43?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21712/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21712      +/-   ##
==========================================
+ Coverage    91.9%   91.91%   +<.01%     
==========================================
  Files         158      158              
  Lines       49690    49689       -1     
==========================================
+ Hits        45670    45671       +1     
+ Misses       4020     4018       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <100%> (ø)` | :arrow_up: |
| #single | `41.95% <43.75%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21712/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.55% <ø> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21712/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.87% <ø> (+0.15%)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21712/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.08% <ø> (-0.11%)` | :arrow_down: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21712/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `94.52% <100%> (+1.76%)` | :arrow_up: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21712/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/timedelta.py](https://codecov.io/gh/pandas-dev/pandas/pull/21712/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YS5weQ==) | `100% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=footer). Last update [7cd2679...32e4269](https://codecov.io/gh/pandas-dev/pandas/pull/21712?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. @TomAugspurger @jorisvandenbossche if any comments. thanks @jbrockmendel 