Remove deprecated function mbedtls_ssl_conf_curves()

[mpg]

The function mbedtls_ssl_conf_curves() has been deprecated in favour of mbedtls_ssl_conf_groups(). One reason it is problematic is because it use mbedtls_ecp_group_id from ecp.h which is going to become private in 1.0/4.0.

This task is to:

• Remove mbedtls_ssl_conf_curves() (declaration and implementation).
• Remove test cases that are specifically about it (the test function conf_curve() in test_suite_ssl can be fully removed). Remove mentions of these tests in analyze_outcomes.py as well.
• I think all test code that's not specifically about it have been migrated to conf_group() already. Double-check and if necessary migrate any leftovers to conf_group().
• Remove mentions of this function from documentation. At this point git grep mbedtls_ssl_conf_curves should only find occurrences in the ChangeLog.
• Add a ChangeLog entry saying this deprecated function was removed and people should use conf_groups instead.
• Remove field curve_list from mbedtls_ssl_config and all places in the code that used it. Normally they should all be guarded with ECP_C && !DEPRECATED_REMOVED already.

This task is development-only.