Fix s_server problem with no-ec

s_server was trying to set the ECDH curve when no-ec was defined. This also
highlighted the fact that the -no_ecdhe option to s_server is broken, and
doesn't make any sense any more (ECDHE is on by default and the only way it
can be disabled is through the cipherstring). Therefore this commit removes
the option.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>