Remove outdated syntax from trait alias pretty printing

[WaffleLapkin]

Given the following program:

#![feature(trait_alias)]
trait A = ?Sized;

fn main() {}

Old output of rustc +nightly ./t.rs -Zunpretty=normal:

#![feature(trait_alias)]
trait A for ? Sized ;

fn main() {}

New output of rustc +a ./t.rs -Zunpretty=normal:

#![feature(trait_alias)]
trait A = ?Sized;

fn main() {}

cc @durka (you've written the FIXME in #45047, see #45047 (comment))

[rustbot]

r? @cjgillot

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[oli-obk]

r? @oli-obk
@bors r+ rollup

[bors]

📌 Commit 12ce0c2 has been approved by oli-obk

It is now in the queue for this repository.

[klensy]

Test to check changes?

[oli-obk]

The pretty printer is not user visible on stable and won't ever be afaik. We only test a few pretty printer things, most are entirely untested and I don't know if it is helpful to increase test coverage for this debugging aid.

[klensy]

There exist https://github.com/rust-lang/rust/tree/master/src/test/pretty, but yes, probably not so necessary.