Rollup merge of rust-lang#69631 - RalfJung:rust-src, r=Mark-Simulacrum

remove non-sysroot sources from rust-src component

See rust-lang#69592 (comment): these were likely added in rust-lang#58269 for the sake of compiler plugins, but those are being entirely phased out, so there is no good reason to ship these sources.

OTOH, @eddyb [wrote](rust-lang#58269 (comment))

> Yeah, my question is why librustc_plugin specifically? Everything else makes sense.

So maybe there is some good reason to keep these? Then we should have a comment explaining that reason.

Cc @eddyb @taeguk @Mark-Simulacrum

src/bootstrap/dist.rs

@@ -1002,8 +1002,6 @@ impl Step for Src {
             "src/tools/rustc-std-workspace-core",
             "src/tools/rustc-std-workspace-alloc",
             "src/tools/rustc-std-workspace-std",
-            "src/librustc",
-            "src/librustc_ast",
         ];
 
         copy_src_dirs(builder, &std_src_dirs[..], &[], &dst_src);