remove obsolete pre-RFC4419 diffie-hellman group exchange

WinSCP uses old D-H group exchange method which is no longer supported by modern OpenSSL.

WinSCP can not connect to these servers unless a workaround is performed.

See the discussion on OpenBSD-misc mailing list at https://marc.info/?t=143079999300001&r=1&w=2

Thanks for your post.

Once PuTTY 0.65 with RFC 4419 support is released, we will include it into WinSCP:
https://git.tartarus.org/?p=simon/putty.git;a=commit;h=62a1bce7cb3ecb98feb57c7f1fd5d55845ce1533