fix: remove CREATE SCHEMA to match CI environment

CI uses schema "public" as default (not "platform"), so creating
a platform schema then tables without prefix puts tables in public
but Prisma looks in platform.

Existing migrations don't create schema - they rely on connection
string's default. Remove CREATE SCHEMA IF NOT EXISTS to match.

autogpt_platform/backend/migrations/20260310_add_llm_registry_schema/migration.sql

@@ -1,6 +1,3 @@
--- CreateSchema (idempotent - safe if already exists)
-CREATE SCHEMA IF NOT EXISTS "platform";
-
 -- CreateEnum
 CREATE TYPE "LlmCostUnit" AS ENUM ('RUN', 'TOKENS');