Add a dummy Store::buildPaths() method

This default implementation of buildPaths() does nothing if all
requested paths are already valid, and throws an "unsupported
operation" error otherwise. This fixes a regression introduced by
c30330df6f in binary cache and legacy
SSH stores.

src/libstore/binary-cache-store.cc

@@ -114,11 +114,6 @@ void BinaryCacheStore::init()
     }
 }
 
-void BinaryCacheStore::notImpl()
-{
-    throw Error("operation not implemented for binary cache stores");
-}
-
 std::shared_ptr<std::string> BinaryCacheStore::getFile(const std::string & path)
 {
     std::promise<std::shared_ptr<std::string>> promise;