Minor: Remove unnecessary name field in ScalarFunctionDefintion

[alamb]

Which issue does this PR close?

Related to / follow on to #8258 from @2010YOUY01

Rationale for this change

While integrating #8258 into our downstream system, I found creating a BuildInScalarFunction to be quite awkward as it had a seemingly duplicated name field. I thought the name field was to handle aliases or something, but it turns out I could remove it easily.

What changes are included in this PR?

Basically change the ScalarFunctionDefinition::BuiltIn to match the others

    BuiltIn {
        fun: built_in_function::BuiltinScalarFunction,
        name: Arc<str>,
    },

to

    BuiltIn(BuiltinScalarFunction),

Are these changes tested?

Covered by existing tests

Are there any user-facing changes?

While this is technically an API change the API hasn't been released so it isn't an additional breaking API change

[alamb]

This is the core change here -- don't keep another string around, but just use the function on demand

[viirya]

Looks more clear to me.