refactor: Make pl.duration non-anonymous

[reswqa]

This closes #13543.

[reswqa]

By convention (leftmost rule), we shouldn't give expression name(duration at this case). So I remove this alias.

[ritchie46]

Not much non-anonymous left I think? 👀

[reswqa]

@ritchie46 Yes, There are only very few left. I have checked, and it falls into two categories:

1. Depending on user defined functions, for example: fold_expr/reduce_exprs/cum_reduce_exprs/cum_fold_exprs.
2. It's not as simple as the other functions to get the output dtype. for example: list.to_struct/list.eval. list.to_struct assigns a rwLock first and then accesses this lock in GetOutput::map_dtype.

   polars/crates/polars-plan/src/dsl/list.rs

   Line 258 in 45e4ce8

   let out_dtype = Arc::new(RwLock::new(None::<DataType>));

As for the first one, IIUC, UDF should not be able to non-anonymous 🤔. As for the second, do you have any thoughts?