[VM/CLI] Remove dartdev.dill

Incompatible VM flags will no longer break the CLI when running from an
AppJIT snapshot, so the fallback logic is no longer required. This CL
thus removes dartdev.dill and the fallback logic.

Relevant past CLs: https://dart-review.googlesource.com/c/sdk/+/157601
and https://dart-review.googlesource.com/c/sdk/+/178300

Fixes https://github.com/dart-lang/sdk/issues/50504

TEST=I tried running `out/ReleaseX64/dart --observe --sound-null-safety test.dart`
and `out/ReleaseX64/dart --observe --no-sound-null-safety test.dart` and
both worked.

Change-Id: I5cdcfbccf71ec557964014fdb80733b4a7c76b4d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/274520
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Derek Xu <derekx@google.com>

utils/dartdev/BUILD.gn

@@ -7,28 +7,11 @@ import("../application_snapshot.gni")
 
 group("dartdev") {
   public_deps = [
-    ":copy_dartdev_kernel",
     ":copy_dartdev_snapshot",
     ":copy_prebuilt_devtools",
   ]
 }
 
-copy("copy_dartdev_kernel") {
-  visibility = [ ":dartdev" ]
-  public_deps = [ ":generate_dartdev_kernel" ]
-  sources = [ "$root_gen_dir/dartdev.dill" ]
-  outputs = [ "$root_out_dir/dartdev.dill" ]
-}
-
-application_snapshot("generate_dartdev_kernel") {
-  dart_snapshot_kind = "kernel"
-  main_dart = "../../pkg/dartdev/bin/dartdev.dart"
-  training_args = []
-  deps = [ "../dds:dds" ]
-  vm_args = [ "--sound-null-safety" ]
-  output = "$root_gen_dir/dartdev.dill"
-}
-
 copy("copy_dartdev_snapshot") {
   visibility = [ ":dartdev" ]
   public_deps = [ ":generate_dartdev_snapshot" ]