Minor: remove unused source file udf.rs

[jonahgao]

Which issue does this PR close?

N/A

Rationale for this change

I have checked that all the UDFs in the file have already been implemented in other modules.
Moreover, udf.rs is not appearing in lib.rs.

What changes are included in this PR?

Are these changes tested?

Yes. By existing tests.

Are there any user-facing changes?

No

[alamb]

Thank you @jonahgao -- this is a great find!

I also double checked a few of these UDFs and found their second copy

🧹

[alamb]

Here is the current copy of the code

datafusion/datafusion/functions-array/src/dimension.rs

Lines 45 to 50 in cd36ee3

	pub(super) struct ArrayDims {
	signature: Signature,
	aliases: Vec<String>,
	}
	impl ArrayDims {

[alamb]

Here is the other copy:

datafusion/datafusion/functions-array/src/string.rs

Lines 113 to 132 in cd36ee3

	pub(super) struct ArrayToString {
	signature: Signature,
	aliases: Vec<String>,
	}
	impl ArrayToString {
	pub fn new() -> Self {
	Self {
	signature: Signature::variadic_any(Volatility::Immutable),
	aliases: vec![
	String::from("array_to_string"),
	String::from("list_to_string"),
	String::from("array_join"),
	String::from("list_join"),
	],
	}
	}
	}
	impl ScalarUDFImpl for ArrayToString {

[jonahgao]

Thank you for double-checking @alamb .