Remove outdated syntax from trait alias pretty printing
Given the following program:
```rust
#![feature(trait_alias)]
trait A = ?Sized;
fn main() {}
```
Old output of `rustc +nightly ./t.rs -Zunpretty=normal`:
```rust
#![feature(trait_alias)]
trait A for ? Sized ;
fn main() {}
```
New output of `rustc +a ./t.rs -Zunpretty=normal`:
```rust
#![feature(trait_alias)]
trait A = ?Sized;
fn main() {}
```
cc `@durka` (you've written the `FIXME` in #45047, see https://github.com/rust-lang/rust/pull/45047#discussion_r144960751)
Matthias Krüger
97008a23fc