Backport of #18709 to 2.0. Removes cleanMigrationsTable() and postUp() from regenerated 1.X migrations. The cleanup method used a hardcoded sylius_migrations table name, which fails when users configure a custom table name via doctrine_migrations.storage.table_storage.table_name.

When upgrading from 1.x to 2.0+, the regenerated migration runs postUp() which tries to delete old 1.x entries from the migrations tracking table. If the user has configured a custom table name (e.g. migration_versions instead of the default sylius_migrations), the query fails because the hardcoded table doesn't exist.

The cleanup was only cosmetic (removing old 1.x migration entries) and not critical - old entries don't cause any issues.