MAINT: Remove combineAdd and combineMult
Deprecated in 0.17.0.

xref #10735
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=h1) Report
> Merging [#15805](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d2f32a0362bbb90c4ab32a454962912901e32080?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15805/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15805      +/-   ##
==========================================
- Coverage   91.02%   90.99%   -0.04%     
==========================================
  Files         143      143              
  Lines       49390    49384       -6     
==========================================
- Hits        44957    44936      -21     
- Misses       4433     4448      +15
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <ø> (-0.41%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :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/15805?src=pr&el=footer). Last update [d2f32a0...f86ad56](https://codecov.io/gh/pandas-dev/pandas/pull/15805?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). thanks!