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_errors/src/diagnostic_impls.rs

@@ -59,7 +59,7 @@ into_diagnostic_arg_using_display!(
     i128,
     u128,
     std::io::Error,
-    std::boxed::Box<dyn std::error::Error>,
+    Box<dyn std::error::Error>,
     std::num::NonZeroU32,
     hir::Target,
     Edition,