Remove the MBEDTLS_SSL_MAX_CONTENT_LEN configuration option

This affects users who use the MBEDTLS_SSL_MAX_CONTENT_LEN option to set the maximum length of incoming and outgoing plaintext fragments, which can save memory by reducing the size of the TLS I/O buffers.

This option is replaced by the more fine-grained options MBEDTLS_SSL_IN_CONTENT_LEN and MBEDTLS_SSL_OUT_CONTENT_LEN that set the maximum incoming and outgoing plaintext fragment lengths, respectively.