MINOR: Remove duplicated blog post for release 15.0.0 (#496)

Closes: #495

This was duplicated in #467
I did forget to push the `git rm` for the old file.

The files are identical:
```diff
$ diff 2024-01-10-15.0.0-release.md 2024-01-21-15.0.0-release.md 
4c4
< date: "2024-01-10 00:00:00"
---
> date: "2024-01-21 00:00:00"

```