Disable vacuuming the DB after garbage collection

Especially in WAL mode on a highly loaded machine, this is not a good
idea because it results in a WAL file of approximately the same size
ad the database, which apparently cannot be deleted while anybody is
accessing it.

nix/libstore/gc.cc

@@ -724,7 +724,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
     }
 
     /* While we're at it, vacuum the database. */
-    if (options.action == GCOptions::gcDeleteDead) vacuumDB();
+    //if (options.action == GCOptions::gcDeleteDead) vacuumDB();
 }