Rid the AST & HIR pretty printer of cruft
Found while working on #119163.
For `trait Trait: ?Sized {}` (semantically malformed), we currently output `trait Trait for ? Sized {}` (sic!) / `trait Trait for ? Sized { }` (sic!) if `-Zunpretty=expanded` / `-Zunpretty=hir` is passed.
`trait Tr for Sized? {}` (#15521) and later also `trait Tr for ?Sized {}` (I guess, #20194) is former Rust syntax. Hence I'm removing these outdated branches.
~~This will conflict with #119163, therefore marking this PR as blocked.~~ Rebased
15dc9f5bee