CLN: Remove unnecessary code
`frozenset()` and `frozenset([])` essentially produce identical results, and thus there is no need to pass an empty list to create an empty `frozenset`.

- [ ] tests added / passed (CI is still running...)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (no new feature)

Hello @suminb! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/accessor.py`](https://github.com/suminb/pandas/blob/eddd73205b79bc580ac05e3148a9c4e54806eae6/pandas/core/accessor.py) !

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/suminb/pandas/blob/eddd73205b79bc580ac05e3148a9c4e54806eae6/pandas/core/generic.py) !

 - There are no PEP8 issues in the file [`pandas/core/groupby/groupby.py`](https://github.com/suminb/pandas/blob/eddd73205b79bc580ac05e3148a9c4e54806eae6/pandas/core/groupby/groupby.py) !

 - In the file [`pandas/tests/dtypes/test_inference.py`](https://github.com/suminb/pandas/blob/eddd73205b79bc580ac05e3148a9c4e54806eae6/pandas/tests/dtypes/test_inference.py), following                are the PEP8 issues :
> [Line 62:18](https://github.com/suminb/pandas/blob/eddd73205b79bc580ac05e3148a9c4e54806eae6/pandas/tests/dtypes/test_inference.py#L62): [E241](https://duckduckgo.com/?q=pep8%20E241) multiple spaces after ','


 ok sure # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23696?src=pr&el=h1) Report
> Merging [#23696](https://codecov.io/gh/pandas-dev/pandas/pull/23696?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/302e4ca1dda7da23822d7c1f40b9d3959aa7fcc9?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/23696/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23696?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23696      +/-   ##
==========================================
+ Coverage   92.24%   92.24%   +<.01%     
==========================================
  Files         161      161              
  Lines       51318    51340      +22     
==========================================
+ Hits        47339    47361      +22     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (ø)` | :arrow_up: |
| #single | `42.34% <100%> (+0.03%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23696?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `96.51% <100%> (ø)` | :arrow_up: |
| [pandas/core/accessor.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWNjZXNzb3IucHk=) | `98.79% <100%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.82% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.74% <0%> (-0.28%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.98% <0%> (-0.09%)` | :arrow_down: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `98.44% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.11% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.47% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `96.14% <0%> (+0.21%)` | :arrow_up: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `96.66% <0%> (+0.4%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/23696/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23696?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/23696?src=pr&el=footer). Last update [302e4ca...eddd732](https://codecov.io/gh/pandas-dev/pandas/pull/23696?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @suminb 