proposal: Remove RFC 5114 MODP DH groups from default proposal

proposal: Remove RFC 5114 MODP DH groups from default proposal

Recent research demonstrates that at least for 1024-bit DH groups, it is
possible to create specially crafted primes having a backdoor. From the
prime itself this is not detectable, creating a perfect NOBUS attack.

http://eprint.iacr.org/2016/961

For the primes defined in RFC 5114 no information is provided on how these
have been selected. In the default proposal we included one of the 2048-bit
primes only, where it is questionable if constructing a backdoored prime is
feasible. Nevertheless, this patch removes the group from the set of default
proposals as well.