Prevent use of RSA+MD5 in TLS 1.2 by default.

Removing RSA+MD5 from the default signature algorithm list
prevents its use by default.

If a broken implementation attempts to use RSA+MD5 anyway the sanity
checking of signature algorithms will cause a fatal alert.
(cherry picked from commit 77a0f740d00ecf8f6b01c0685a2f858c3f65a3dd)