Remove `CIRRUS_TASK_NAME` from what I can tell, is always omitted on `LUCI` (#161391)

Towards https://github.com/flutter/flutter/issues/161387.

Example build, no `CIRRUS_TASK_NAME`:

<https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8726279068923920961/+/u/OS_info/l_execution_details>

dev/bots/test.dart

diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index fe85b50..1d20a0e 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -22,10 +22,6 @@
 // supported, or run with `--dry-run` to get a list of tests that _would_ have
 // been executed.
 //
-// If the CIRRUS_TASK_NAME environment variable exists, it is used to determine
-// the shard and sub-shard, by parsing it in the form shard-subshard-platform,
-// ignoring the platform.
-//
 // For local testing you can just set the SHARD and SUBSHARD environment
 // variables. For example, to run all the framework tests you can just set
 // SHARD=framework_tests. Some shards support named subshards, like
@@ -77,8 +73,6 @@
 /// if such flags are provided to `test.dart`.
 final Map<String, String> localEngineEnv = <String, String>{};
 
-const String CIRRUS_TASK_NAME = 'CIRRUS_TASK_NAME';
-
 /// When you call this, you can pass additional arguments to pass custom
 /// arguments to flutter test. For example, you might want to call this
 /// script with the parameter --local-engine=host_debug_unopt to
@@ -125,9 +119,6 @@
     if (dryRunArgSet) {
       enableDryRun();
     }
-    if (Platform.environment.containsKey(CIRRUS_TASK_NAME)) {
-      printProgress('Running task: ${Platform.environment[CIRRUS_TASK_NAME]}');
-    }
     final WebTestsSuite webTestsSuite = WebTestsSuite(flutterTestArgs);
     await selectShard(<String, ShardRunner>{
       'add_to_app_life_cycle_tests': addToAppLifeCycleRunner,

dev/bots/utils.dart

diff --git a/dev/bots/utils.dart b/dev/bots/utils.dart
index 4f573cc..613a681 100644
--- a/dev/bots/utils.dart
+++ b/dev/bots/utils.dart
@@ -69,7 +69,6 @@
 const String kShardKey = 'SHARD';
 const String kSubshardKey = 'SUBSHARD';
 const String kTestHarnessShardName = 'test_harness_tests';
-const String CIRRUS_TASK_NAME = 'CIRRUS_TASK_NAME';
 
 /// Environment variables to override the local engine when running `pub test`,
 /// if such flags are provided to `test.dart`.
@@ -701,12 +700,7 @@
   int positionInTaskName,
 ) async {
   try {
-    String? item = Platform.environment[key];
-    if (item == null && Platform.environment.containsKey(CIRRUS_TASK_NAME)) {
-      final List<String> parts = Platform.environment[CIRRUS_TASK_NAME]!.split('-');
-      assert(positionInTaskName < parts.length);
-      item = parts[positionInTaskName];
-    }
+    final String? item = Platform.environment[key];
     if (item == null) {
       for (final String currentItem in items.keys) {
         printProgress('$bold$key=$currentItem$reset');