[VM] Remove support for --no-sound-null-safety mode

This CL removes support for --no-sound-null-safety option, the VM will
only run sound null safety code.

First part of deleting support for legacy and unsound null safe code.

TEST=CI

Change-Id: I9dbed10a18ef102533d6af516ff5075297d17b74
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/357183
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

utils/dartdev/BUILD.gn

@@ -28,7 +28,7 @@ application_snapshot("generate_dartdev_snapshot") {
     "../dtd:dtd",
   ]
 
-  vm_args = [ "--sound-null-safety" ]
+  vm_args = []
   output = "$root_gen_dir/dartdev.dart.snapshot"
 }