Rollup merge of #148673 - fmease:del-dyn_star-remnant, r=JonathanBrouwer

Remove a remnant of `dyn*` from the parser

Follow-up to rust-lang/rust#146664 and rust-lang/rust#143036.

`is_explicit_dyn_type` still checked for `TokenKind::Star` which made no sense now that `dyn*` is no more.

Removing it doesn't represent a functional change and merely affects diagnostics. That's because the check only dictated whether to interpret `dyn` as the start of a trait object type in Rust 2015 (where this identifier is only a *contextual* keyword). However, we would still fail at the `*` later on as it doesn't start a bound.

While at it, I also took the time to clean up in the vicinity.

Author: Zalathar