Don't use RSA+MD5 with TLS 1.2

Since the TLS 1.2 supported signature algorithms extension is less
sophisticaed in OpenSSL 1.0.1 this has to be done in two stages.

RSA+MD5 is removed from supported signature algorithms extension:
any compliant implementation should never use RSA+MD5 as a result.

To cover the case of a broken implementation using RSA+MD5 anyway
disable lookup of MD5 algorithm in TLS 1.2.