isalink: Prune isabelle db logic

isa-client supports basic interoperability with the isabelle.db SQLite database used by standard Isabelle servers/clients.

This is however not really used by us in practice because isabelle.db doesn't support server addresses and always assumes all servers live on localhost. We could extend the isabelle.db but we have already forgone usage of the standard Isabelle client anyway.

Given that ISM doesn't support this either, nor is likely to run on localhost, the code is unused.

NOTE: This code also adds, via the modernc SQLite dependency, a significant compile time overhead.