fix: remove unused DPO parameters from schema and tests (#2988)

# What does this PR do?

I removed these DPO parameters from the schema in [this
PR](https://github.com/meta-llama/llama-stack/pull/2804), but I may not
have done it correctly, since they were reintroduced in [this
commit](cb7354a9ce (diff-4e9a8cb358213d6118c4b6ec2a76d0367af06441bf0717e13a775ade75e2061dR15081))—likely
due to a pre-commit hook.

I've made the changes again, and the pre-commit hook automatically
updated the spec sheet.

docs/_static/llama-stack-spec.html

@@ -15078,22 +15078,6 @@
             "DPOAlignmentConfig": {
                 "type": "object",
                 "properties": {
-                    "reward_scale": {
-                        "type": "number",
-                        "description": "Scaling factor for the reward signal"
-                    },
-                    "reward_clip": {
-                        "type": "number",
-                        "description": "Maximum absolute value for reward clipping"
-                    },
-                    "epsilon": {
-                        "type": "number",
-                        "description": "Small value added for numerical stability"
-                    },
-                    "gamma": {
-                        "type": "number",
-                        "description": "Discount factor for future rewards"
-                    },
                     "beta": {
                         "type": "number",
                         "description": "Temperature parameter for the DPO loss"
@@ -15106,10 +15090,6 @@
                 },
                 "additionalProperties": false,
                 "required": [
-                    "reward_scale",
-                    "reward_clip",
-                    "epsilon",
-                    "gamma",
                     "beta",
                     "loss_type"
                 ],