[Core][Misc] Clean up ProfileExecuteDuration (#6461)

### What this PR does / why we need it?
This PR removes the custom `ProfileExecuteDuration` utility and its
usages across the codebase. This utility was used for profiling
execution duration of different stages in the inference process. It is
replaced by the standard `vllm.v1.utils.record_function_or_nullcontext`,
which integrates with PyTorch's profiler.

This change simplifies the code by removing a custom implementation in
favor of an upstream utility, improving maintainability. Associated
documentation and tests for `ProfileExecuteDuration` are also removed.

### Does this PR introduce _any_ user-facing change?
`VLLM_ASCEND_MODEL_EXECUTE_TIME_OBSERVE` env is removed now.

### How was this patch tested?
CI passed. The changes are a cleanup and replacement with a standard
utility. Existing tests cover the functionality. The removed feature had
its own tests which are also removed.

Related RFC: #5304

- vLLM version: v0.14.1
- vLLM main:
dc917cceb8

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

tests/ut/test_utils.py

@@ -246,56 +246,3 @@ class TestUtils(TestBase):
         utils.register_ascend_customop()
         self.assertEqual(mock_customop.register_oot.call_count,
                          len(REGISTERED_ASCEND_OPS))
-
-
-class TestProfileExecuteDuration(TestBase):
-
-    def setUp(self):
-        utils.ProfileExecuteDuration._instance = None
-        utils.ProfileExecuteDuration._observations = []
-        utils.ProfileExecuteDuration._lock = Lock()
-
-    def test_singleton_creation(self):
-        instance1 = utils.ProfileExecuteDuration()
-        self.assertIsNotNone(instance1)
-        self.assertIs(instance1, utils.ProfileExecuteDuration._instance)
-
-        instance2 = utils.ProfileExecuteDuration()
-        self.assertIs(instance1, instance2)
-
-    def test_thread_safety(self):
-        from threading import Thread
-
-        instances = []
-
-        def create_instance():
-            instances.append(utils.ProfileExecuteDuration())
-
-        threads = [Thread(target=create_instance) for _ in range(10)]
-        for t in threads:
-            t.start()
-        for t in threads:
-            t.join()
-
-        first_instance = instances[0]
-        for instance in instances[1:]:
-            self.assertIs(first_instance, instance)
-
-    def test_atexit_registration(self):
-        with mock.patch('atexit.register') as mock_register:
-            instance = utils.ProfileExecuteDuration()
-            mock_register.assert_called_once_with(instance.destroy)
-
-    def test_lock_usage(self):
-        original_lock = utils.ProfileExecuteDuration._lock
-
-        with mock.patch.object(utils.ProfileExecuteDuration,
-                               '_lock',
-                               wraps=original_lock) as mock_lock:
-            utils.ProfileExecuteDuration()
-            mock_lock.__enter__.assert_called()
-            mock_lock.__exit__.assert_called()
-
-    def test_observations_initialization(self):
-        instance = utils.ProfileExecuteDuration()
-        self.assertEqual(instance._observations, [])