refactor: Move stopafterblockimport handling out of blockstorage

This has the benefit of moving the StartShutdown call out of the
blockstorage file and thus out of the kernel's responsibility. The user
can now decide if he wants to start shutdown / interrupt after a block
import or not.

src/node/blockmanager_args.cpp

@@ -32,7 +32,6 @@ util::Result<void> ApplyArgsManOptions(const ArgsManager& args, BlockManager::Op
     opts.prune_target = nPruneTarget;
 
     if (auto value{args.GetBoolArg("-fastprune")}) opts.fast_prune = *value;
-    if (auto value{args.GetBoolArg("-stopafterblockimport")}) opts.stop_after_block_import = *value;
 
     return {};
 }