fix(coderabbit): align config to schema v2

Remove unsupported keys to avoid parser warnings and unblock clean bot reviews.

Co-authored-by: Codex <noreply@openai.com>

Author: KooshaPari

.coderabbit.yaml

@@ -6,17 +6,3 @@ reviews:
   auto_review:
     enabled: true
     auto_incremental_review: true
-
-pre_merge_checks:
-  docstrings:
-    mode: warning
-  title:
-    mode: warning
-  description:
-    mode: warning
-  issue_assessment:
-    mode: warning
-
-pr_validation:
-  block_on:
-    severity: info