Remove assertExecuteOnStartThread from AbstractSearchAsyncAction (elastic#121922)

This is a really strange assertion. I get that it tries to make sure we
skip unavailable without forking but this makes extending the
AbstractSearchAsyncAction cleanly for batched execution needlessly hard
and some of the assertion is dead code already because can-match isn't
going through this codepath anymore.

-> lets remove it, the code is simple enough now to follow that there's
no forking here IMO

Author: original-brownbear

server/src/main/java/org/elasticsearch/action/search/AbstractSearchAsyncAction.java

@@ -256,32 +256,6 @@ void skipShard(SearchShardIterator iterator) {
         successfulShardExecution(iterator);
     }
 
-    private static boolean assertExecuteOnStartThread() {
-        // Ensure that the current code has the following stacktrace:
-        // AbstractSearchAsyncAction#start -> AbstractSearchAsyncAction#executePhase -> AbstractSearchAsyncAction#performPhaseOnShard
-        final StackTraceElement[] stackTraceElements = Thread.currentThread().getStackTrace();
-        assert stackTraceElements.length >= 6 : stackTraceElements;
-        int index = 0;
-        assert stackTraceElements[index++].getMethodName().equals("getStackTrace");
-        assert stackTraceElements[index++].getMethodName().equals("assertExecuteOnStartThread");
-        assert stackTraceElements[index++].getMethodName().equals("failOnUnavailable");
-        if (stackTraceElements[index].getMethodName().equals("performPhaseOnShard")) {
-            assert stackTraceElements[index].getClassName().endsWith("CanMatchPreFilterSearchPhase");
-            index++;
-        }
-        assert stackTraceElements[index].getClassName().endsWith("AbstractSearchAsyncAction");
-        assert stackTraceElements[index++].getMethodName().equals("run");
-
-        assert stackTraceElements[index].getClassName().endsWith("AbstractSearchAsyncAction");
-        assert stackTraceElements[index++].getMethodName().equals("executePhase");
-
-        assert stackTraceElements[index].getClassName().endsWith("AbstractSearchAsyncAction");
-        assert stackTraceElements[index++].getMethodName().equals("start");
-
-        assert stackTraceElements[index].getClassName().endsWith("AbstractSearchAsyncAction") == false;
-        return true;
-    }
-
     private void performPhaseOnShard(final int shardIndex, final SearchShardIterator shardIt, final SearchShardTarget shard) {
         if (throttleConcurrentRequests) {
             var pendingExecutions = pendingExecutionsPerNode.computeIfAbsent(
@@ -323,7 +297,6 @@ public void onFailure(Exception e) {
     }
 
     private void failOnUnavailable(int shardIndex, SearchShardIterator shardIt) {
-        assert assertExecuteOnStartThread();
         SearchShardTarget unassignedShard = new SearchShardTarget(null, shardIt.shardId(), shardIt.getClusterAlias());
         onShardFailure(shardIndex, unassignedShard, shardIt, new NoShardAvailableActionException(shardIt.shardId()));
     }