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GH-44558: [Release][Website] Remove needless "Apache Arrow ${VERSION}" section (#44559)

### Rationale for this change

`dev/release/post-04-website.sh` generates wrong section levels:

```markdown
...
## Changelog
## Apache Arrow 18.0.0 (2024-10-28 07:00:00+00:00)
...
### Bug Fixes
...
### New Features and Improvements
...
```

### What changes are included in this PR?

Remove the `## Apache Arrow ${VERSION}` section:

```markdown
...
## Changelog
...
### Bug Fixes
...
### New Features and Improvements
...
```

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #44558

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>