[gardening] Remove null safety flag from newly added test

This fixes an issue in our testing infrastructure: If // VMOptions
swiches the null safety mode explicitly, all tools/test.py runs that
require multiple steps (e.g AOT that compiles+runs or simarm that
compiles to kernel + runs) would fail with mismatch in null safety mode.

Instead rely on our test matrix to run this test in all null safety
configurations.

TEST=ci

Change-Id: I6e2f6963351a1a6c071cefb4444e64d2904c63f2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249001
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>

runtime/tests/vm/dart/regress_49279_test.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// VMOptions=--null-assertions --no-sound-null-safety --enable-asserts
+// VMOptions=--null-assertions --enable-asserts
 
 void main() {
   try {

runtime/tests/vm/dart_2/regress_49279_test.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// VMOptions=--null-assertions --no-sound-null-safety --enable-asserts
+// VMOptions=--null-assertions --enable-asserts
 
 void main() {
   try {