DOC: Removing old release.rst

[datapythonista]

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]

Codecov Report

Merging #24427 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	43% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #24427 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	43% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2a09706...4ceea00. Read the comment docs.

[TomAugspurger]

Thanks.