Remove use of $NIX_HELD_LOCKS

This variable was once used to communicate already acquired store path
locks between Nix and the build hook, but this hasn't been the case
since 9bcb4d2dd9. So let's get rid of
it.

src/libstore/local-store.cc

@@ -110,7 +110,6 @@ LocalStore::LocalStore(
     , schemaPath(dbDir + "/schema")
     , tempRootsDir(stateDir + "/temproots")
     , fnTempRoots(fmt("%s/%d", tempRootsDir, getpid()))
-    , locksHeld(tokenizeString<PathSet>(getEnv("NIX_HELD_LOCKS").value_or("")))
 {
     auto state(_state.lock());
     state->stmts = std::make_unique<State::Stmts>();