DOC: Removing old release.rst
In #21599 we refactored the whatsnew and release pages. The old `release.rst` was substituted by a `releases.rst`. But wasn't removed, even if it's not rendered by sphinx or linked anywhere.

Deleting the old unused `release.rst` here, which causes several warning because of duplicate labels.

CC: @TomAugspurger 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=h1) Report
> Merging [#24427](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2a097061700e2441349cafd89348c9fc8d14ba31?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/24427/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24427   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         163      163           
  Lines       51954    51954           
=======================================
  Hits        47957    47957           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24427?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/24427?src=pr&el=footer). Last update [2a09706...4ceea00](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=h1) Report
> Merging [#24427](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2a097061700e2441349cafd89348c9fc8d14ba31?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/24427/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24427   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         163      163           
  Lines       51954    51954           
=======================================
  Hits        47957    47957           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24427?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/24427?src=pr&el=footer). Last update [2a09706...4ceea00](https://codecov.io/gh/pandas-dev/pandas/pull/24427?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks.