Remove deprecated function mbedtls_x509write_crt_set_serial()

[mpg]

The function mbedtls_x509write_crt_set_serial() has been deprecated in favour of mbedtls_x509write_crt_set_serial_raw(). On reason it's problematic is because it has an argument of type mbedtls_mpi from bignum.h which is going to be private in 1.0/4.0.

This task is to:

• Remove this function (declaration and definition).
• Remove tests that are specifically about it. (Test that are not specifically about it should have already been migrate to mbedtls_x509write_crt_set_serial_raw() but still double-check.)
• Remove mentions in the documentation. At this point git grep -w mbedtls_x509write_crt_set_serial should only find matches in the ChangeLog.
• Add a ChangeLog entry about the removal.

This task is development-only.