Addresses P400136088 Bug 1 and V2146375387 (Triton path).

Three changes:

1. check_integrity.py: Switch from HMAC-SHA256 to plain SHA-256.
   - Remove generate_secret_key() — no longer needed
   - compute_hash() now uses hashlib.sha256() instead of hmac.new()
   - perform_integrity_check() no longer reads SAGEMAKER_SERVE_SECRET_KEY
     from environment

2. triton/model.py: Add integrity check in initialize() BEFORE
   cloudpickle deserialization. Previously the handler called
   cloudpickle.load() with no verification (acknowledged by a TODO
   comment). Now reads the file into a buffer, runs
   perform_integrity_check(), then deserializes with cloudpickle.loads().

3. triton/server.py: Remove SAGEMAKER_SERVE_SECRET_KEY from container
   environment variables in both local and SageMaker deployment modes.
   The key is no longer needed since integrity checking uses plain
   SHA-256.

4. model_builder_utils.py: Update _hmac_signing() to use plain SHA-256
   and stop generating/storing a secret key. Remove generate_secret_key
   import.

The integrity check still detects accidental corruption of model
artifacts in S3. The HMAC was providing a false sense of security since
the key was exposed via DescribeModel/DescribeEndpointConfig APIs.