As presented in STM32MPU OP-TEE Overview article, STM32MPU embeds OP-TEE either for system services or for system and secure services. Platforms that do not need the secure services still need to embed OP-TEE for the system services, due to the hardware architecture.
In order to disable OP-TEE secure services and embeds only the system services, one shall build OP-TEE OS with configuration CFG_STM32MP_PROFILE=system_services. Detailed information can be found in the system services profile article.