CLEAN: Removing outdated and unused CI files
Removing outdated and unused files.
- `ci/print_versions.py` doesn't work, as the used path has changed. It's not used anywhere in the code, and when needed, `python -c "import pandas; pandas.show_versions();"` is used instead.
- `ci/README.txt` it's outdated and incomplete. After updating the CI and making it more compact, I'll try to add a section to the contributing page in the docs, about what we have. This should be more useful than this file.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24004?src=pr&el=h1) Report
> Merging [#24004](https://codecov.io/gh/pandas-dev/pandas/pull/24004?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dc8d35aa53d496b651f5e1ab4cb2604e9f7236c7?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24004/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24004?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24004   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         161      161           
  Lines       51549    51549           
=======================================
  Hits        47586    47586           
  Misses       3963     3963
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.44% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24004?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/24004?src=pr&el=footer). Last update [dc8d35a...a8dcda9](https://codecov.io/gh/pandas-dev/pandas/pull/24004?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!