x509: remove obsolete TODO comment
x509: remove obsolete TODO comment

- basicContraints checks are done during verification
- there is no need to set extensions that are not present to default values,
  as the code using the extension will check if it was present using
ext_types. (And default values would not make sense anyway.)

ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls