Bug#391837: Remove RSA_EXPORT support

[Florian Weimer]

* Andreas Metzler:

> I might just be too stupid today, but afaict the patch does not change
> generation of dh_params at all, it is still done exactly the same way
> as before, by calling gnutls_dh_params_generate2().

As far as I can see, "DH parameters" is a misnomer; Exim stores both
the RSA_EXPORT key and the actual DH parameters in the same file.