Remove MaximumFragmentLength (MFL) query API

This affects users which use the MFL query APIs mbedtls_ssl_get_{input,output}_max_frag_len() to infer upper bounds on the plaintext size of incoming and outgoing record.

Users should switch to mbedtls_ssl_get_max_{in,out}_record_payload() instead, which also provides such upper bounds but takes more factors than just the MFL configuration into account.