Remove SIL configs from dub.sdl.

[otrho]

Having the SIL registration code in this repo introduces dependencies on sil-lang which are marked as optional in dub.sdl because there's a dub bug or something if they're not.

Ideally we should remove all the SIL stuff from here and keep it as just an IMAP D library and the SIL stuff can go in a dedicated plugin library hosted elsewhere which depends on this one.

[Laeeth]

Symmetry is welcome to remove the JMAP functionality but if it does so then I will have to fork it and put it up on code.dlang.org with JMAP and there will then be two forks with duplicated code, which I don't think is ideal.

I think it would be better either to open source Variable or to modify the code so it doesn't depend on Variable and you have an adaptor in the plugin.