Ensure that Perl processes delete their entry in the temproots directory

By moving the destructor object to libstore.so, it's also run when
download-using-manifests and nix-prefetch-url exit.  This prevents
them from cluttering /nix/var/nix/temproots with stale files.

src/nix-worker/nix-worker.cc

@@ -584,8 +584,6 @@ static void performOp(unsigned int clientVersion,
 
 static void processConnection()
 {
-    RemoveTempRoots removeTempRoots __attribute__((unused));
-
     canSendStderr = false;
     myPid = getpid();    
     writeToStderr = tunnelStderr;