ty: remove obsolete pretty printer
Fixes#61139.
This PR removes the obsolete printer and replaces all uses of it with `FmtPrinter`. Of the replaced uses, all but one use was in `debug!` logging, two cases were notable:
- `MonoItem::to_string` is used in `-Z print-mono-items` and therefore affects the output of all codegen-units tests (which have been updated).
- `DefPathBasedNames` was used in `librustc_codegen_llvm/type_of.rs` with `LLVMStructCreateNamed` and that'll now get different values, but nothing will break as a result of this.
cc @eddyb (whom I've discussed this with)
8ed5cb56b5