GH-53730: [C++] Simplify destructor of ArrowLog

[taepper]

Rationale for this change

@elfring correctly identified in #53730 that we perform an unnecessary null check before invoking delete.

Calling delete on a nullptr is completely safe.

What changes are included in this PR?

This simplifies the destructor of ArrowLog.

Additionally:

• explicitly setting the member to nullptr in the destructor is unnecessary
• my static analyzer also noticed that we can replace the reinterpret_cast with a safer static_cast

Are these changes tested?

Yes

Are there any user-facing changes?

No

Was AI used for this PR?

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++] Remove an unnecessary null pointer check #53730

[github-actions]

⚠️ GitHub issue #53730 has been automatically assigned in GitHub to PR creator.

[taepper]

Small simplification and issue clean-up @pitrou @HuaHuaY

[elfring]

Thanks for your small source code improvement.