remove daytime attr, move getstate and setstate to base class
If/when #18224 is revived to make `_BaseOffset` a `cdef class`, we'll need to move `__getstate__` and `__setstate__` into the base class (and then make some changes).  This moves the two methods pre-emptively so that we'll have a smaller diff for the next steps.

Also removes an attr `self.daytime` that is not used anywhere else.
lgtm. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21533?src=pr&el=h1) Report
> Merging [#21533](https://codecov.io/gh/pandas-dev/pandas/pull/21533?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6131a598a614b93e70a28b36d35819cb6081965b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21533/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21533?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21533      +/-   ##
==========================================
- Coverage   91.92%   91.92%   -0.01%     
==========================================
  Files         153      153              
  Lines       49594    49570      -24     
==========================================
- Hits        45590    45566      -24     
  Misses       4004     4004
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.81% <ø> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21533?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21533/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.19% <ø> (-0.06%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21533?src=pr&el=footer). Last update [6131a59...5c6dfbb](https://codecov.io/gh/pandas-dev/pandas/pull/21533?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!