rustc: remove unnecessary extern_prelude logic from ty::item_path

[Dylan-DPC-zz]

From #56655 :

The checks added in 02357e4 effectively turned crate::std into std, but they were too general (affecting any crate::foo where foo was in the extern prelude, not just extern crates), and unnecessary, as only the extern crates created by "std injection" need any special-casing.
Since this only affects the user-facing "relative" mode, it shouldn't have interactions with linking, and the only observable effect should be sometimes-shorter paths in diagnostics.

Creating this issue as a tracking issue since the PR is closed due to inactivity.