`Catalog.struct_tree_root` is never used

[notxvilka]

No matter, reading or writing, the field is never used:

pdf/pdf on  master took 11.5s                                                                                                                                                                20:02:17 via 🦀 1.84.1
$ rg struct_tree_root
src/build.rs
147:            struct_tree_root: None,

src/object/types.rs
160:    pub struct_tree_root: Option<StructTreeRoot>,