dns-client.opam: remove unused lwt dependency for the pure client

[cfcs]

fixes #271, thanks to @copy for reporting.

[hannesm]

indeed, the current opam package layout is to have a single dns-client and sublibraries (dns-client.unix, dns-client.lwt, dns-client.mirage) and all dependencies for all backends are part of dns-client.opam. I can see three ways forward:

• continue as is (they are only build time dependencies, not runtime ones)
• split into dns-client-unix, dns-client-lwt, dns-client-mirage opam packages (more load on the opam solver)
• use optional dependencies (disadvantage is that something to use dns-client-mirage needs to list dns-client and all dependencies needed for dns-client.mirage).