[SPARK-48408][SQL] Simplify `date_format` & `from_unixtime`

[panbingkun]

What changes were proposed in this pull request?

The pr aims to simplify date_format & from_unixtime.

Why are the changes needed?

Make the code look more concise.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

• Existed UT
• Pass GA.

Was this patch authored or co-authored using generative AI tooling?

No.