[Cleanup] Removes deprecated --gc_at_instance_allocation.

The flag only appears in two tests and a comment.
Flag was removed in https://dart-review.googlesource.com/c/sdk/+/83120

Change-Id: I2a5d828c8ec7909a3d16579f9a9771176aabb65e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136974
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Clement Skau <cskau@google.com>

runtime/vm/heap/heap.h

@@ -63,7 +63,7 @@ class Heap {
     kExternal,   // Dart_NewWeakPersistentHandle
     kIdle,       // Dart_NotifyIdle
     kLowMemory,  // Dart_NotifyLowMemory
-    kDebugging,  // service request, --gc_at_instance_allocation, etc.
+    kDebugging,  // service request, etc.
   };
 
   // Pattern for unused new space and swept old space.

tests/language_2/vm/regress_22621_vm_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.
 // Test that BoxAllocationSlowPath for Mint emits stackmap in unoptimized code.
-// VMOptions=--gc_at_instance_allocation=_Mint --inline_alloc=false
+// VMOptions=--inline_alloc=false
 
 main() {
   var re = new RegExp(r"IsolateStubs (.*)");

tests/standalone_2/slowpath_safepoints_test.dart

@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 // Test that safepoints associated with slowpaths don't mark non-existing values
 // alive.
-// VMOptions=--optimization-counter-threshold=5 --no-inline_alloc --gc_at_instance_allocation=_Double --no-background-compilation
+// VMOptions=--optimization-counter-threshold=5 --no-inline_alloc --no-background-compilation
 
 class C {
   final next;