Remove the macro rule `unary_scalar_expr` from `expr_fn.rs`

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
https://github.com/apache/arrow-datafusion/blob/bcd624855778384ee27648161de73951e3fb6ea1/datafusion/expr/src/expr_fn.rs#L308-L331
We can use the scalar_expr to create unary scalar expr functions, so that the macro rule unary_scalar_functions is not necessary.
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[HaoYang670]

Also, the doc in scalar_expr isn't always meaningful. We should also update it.