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.yaml

@@ -11163,20 +11163,6 @@ components:
     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
@@ -11186,10 +11172,6 @@ components:
           description: The type of loss function to use for DPO
       additionalProperties: false
       required:
-        - reward_scale
-        - reward_clip
-        - epsilon
-        - gamma
         - beta
         - loss_type
       title: DPOAlignmentConfig