[build] Remove dart_host_toolchain.

Consensus seems to be that there should be different mechanism to support host-targeting-host vs host-targeting-target configuration: comparing toolchains names won't work for that.
Also, dart_host_toolchain was set up to be used by Fuchsia, but it is no longer being used.

Change-Id: Ic2e63d8cef00b18bf6866122199027459eaf32c6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/118910
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

utils/application_snapshot.gni

@@ -78,11 +78,10 @@ template("_application_snapshot") {
   # Build the kernel file using the prebuilt VM to speed up the debug and
   # simulator builds.
   prebuilt_dart_action(target_name + "_dill") {
-    deps =
-        extra_deps + [
-          "$_dart_root/runtime/vm:kernel_platform_files($dart_host_toolchain)",
-          "$_dart_root/runtime/vm:vm_platform",
-        ]
+    deps = extra_deps + [
+             "$_dart_root/runtime/vm:kernel_platform_files($host_toolchain)",
+             "$_dart_root/runtime/vm:vm_platform",
+           ]
     gen_kernel_script = "$_dart_root/pkg/vm/bin/gen_kernel.dart"
     platform_dill = "$root_out_dir/vm_platform_strong.dill"