Create default Store::narFromPath implementation in terms of getFSAccessor

This is a good default (the methods that allow for an arbitrary choice
of source accessor are generally preferable both to implement and to
use). And it also pays its way by allowing us to delete *both* the
`DummyStore` and `LocalStore` implementations.

src/libstore/ssh-store.cc

@@ -143,7 +143,7 @@ struct MountedSSHStore : virtual SSHStore, virtual LocalFSStore
 
     void narFromPath(const StorePath & path, Sink & sink) override
     {
-        return LocalFSStore::narFromPath(path, sink);
+        return Store::narFromPath(path, sink);
     }
 
     ref<SourceAccessor> getFSAccessor(bool requireValidPath) override