Rollup merge of #105537 - kadiwa4:remove_some_imports, r=fee1-dead

compiler: remove unnecessary imports and qualified paths

Some of these imports were necessary before Edition 2021, others were already in the prelude.

I hope it's fine that this PR is so spread-out across files :/

compiler/rustc_expand/src/base.rs

@@ -34,7 +34,6 @@ use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, FileName, RealFileName, Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 
-use std::default::Default;
 use std::iter;
 use std::path::PathBuf;
 use std::rc::Rc;