Remove dead code RemoteStore::sameMachine

It has been dead code since 9747ea84b4.

src/libstore/ssh-store.cc

@@ -49,9 +49,6 @@ public:
         return *uriSchemes().begin() + "://" + host;
     }
 
-    bool sameMachine() override
-    { return false; }
-
     // FIXME extend daemon protocol, move implementation to RemoteStore
     std::optional<std::string> getBuildLogExact(const StorePath & path) override
     { unsupported("getBuildLogExact"); }