refactor: remove unused sysroot_host_libdir

Copy from <#10469 (comment)>:

> I've never been entirely clear why it does this. #4006 didn't really
> explain why it added the corresponding host_dylib_path. I can't envision
> a scenario where it matters. I think compiler plugins and proc-macros
> should load just fine, since libstd.so should already be loaded by the
> compiler. Also, rustc uses rpath these days, and on Windows libstd.so is
> placed in the bin directory which will be searched first anyways.
>
> On balance, I think it should be safe to just remove sysroot_host_libdir.
> I can't come up with a scenario where it matters, at least on
> windows/macos/linux. One issue is that this is most likely to affect
> plugins, but those are deprecated and I think only Servo was the real
> holdout. A concern is that nobody is going to test this use case before
> it hits stable.

Also,

* compiler plugins were removed rust-lang/rust#116412
* servo has moved off from plugins: servo/servo#30508

So should generally be fine.

src/cargo/core/compiler/build_context/target_info.rs

@@ -48,9 +48,6 @@ pub struct TargetInfo {
     support_split_debuginfo: Vec<String>,
     /// Path to the sysroot.
     pub sysroot: PathBuf,
-    /// Path to the "lib" or "bin" directory that rustc uses for its dynamic
-    /// libraries.
-    pub sysroot_host_libdir: PathBuf,
     /// Path to the "lib" directory in the sysroot which rustc uses for linking
     /// target libraries.
     pub sysroot_target_libdir: PathBuf,
@@ -224,19 +221,17 @@ impl TargetInfo {
                 return error_missing_print_output("sysroot", &process, &output, &error);
             };
             let sysroot = PathBuf::from(line);
-            let sysroot_host_libdir = if cfg!(windows) {
-                sysroot.join("bin")
-            } else {
-                sysroot.join("lib")
+            let sysroot_target_libdir = {
+                let mut libdir = sysroot.clone();
+                libdir.push("lib");
+                libdir.push("rustlib");
+                libdir.push(match &kind {
+                    CompileKind::Host => rustc.host.as_str(),
+                    CompileKind::Target(target) => target.short_name(),
+                });
+                libdir.push("lib");
+                libdir
             };
-            let mut sysroot_target_libdir = sysroot.clone();
-            sysroot_target_libdir.push("lib");
-            sysroot_target_libdir.push("rustlib");
-            sysroot_target_libdir.push(match &kind {
-                CompileKind::Host => rustc.host.as_str(),
-                CompileKind::Target(target) => target.short_name(),
-            });
-            sysroot_target_libdir.push("lib");
 
             let support_split_debuginfo = {
                 // HACK: abuse `--print=crate-name` to use `___` as a delimiter.
@@ -303,7 +298,6 @@ impl TargetInfo {
                 crate_type_process,
                 crate_types: RefCell::new(map),
                 sysroot,
-                sysroot_host_libdir,
                 sysroot_target_libdir,
                 rustflags,
                 rustdocflags: extra_args(

src/cargo/core/compiler/compilation.rs

@@ -74,9 +74,6 @@ pub struct Compilation<'gctx> {
     /// May be for the host or for a specific target.
     pub deps_output: HashMap<CompileKind, PathBuf>,
 
-    /// The path to the host libdir for the compiler used
-    sysroot_host_libdir: PathBuf,
-
     /// The path to libstd for each target
     sysroot_target_libdir: HashMap<CompileKind, PathBuf>,
 
@@ -128,11 +125,6 @@ impl<'gctx> Compilation<'gctx> {
             native_dirs: BTreeSet::new(),
             root_output: HashMap::new(),
             deps_output: HashMap::new(),
-            sysroot_host_libdir: bcx
-                .target_data
-                .info(CompileKind::Host)
-                .sysroot_host_libdir
-                .clone(),
             sysroot_target_libdir: get_sysroot_target_libdir(bcx)?,
             tests: Vec::new(),
             binaries: Vec::new(),
@@ -282,7 +274,6 @@ impl<'gctx> Compilation<'gctx> {
         let mut search_path = Vec::new();
         if is_rustc_tool {
             search_path.push(self.deps_output[&CompileKind::Host].clone());
-            search_path.push(self.sysroot_host_libdir.clone());
         } else {
             search_path.extend(super::filter_dynamic_search_path(
                 self.native_dirs.iter(),