mempool: Pass in -mempoolexpiry instead of referencing gArgs

- Store the mempool expiry (-mempoolexpiry) in CTxMemPool as a
  std::chrono::seconds member.

- Remove the requirement to explicitly specify a mempool expiry for
  LimitMempoolSize(...), just use the newly-introduced member.

- Remove all now-unnecessary instances of:
    std::chrono::hours{gArgs.GetIntArg("-mempoolexpiry", DEFAULT_MEMPOOL_EXPIRY)}

src/mempool_args.cpp

@@ -15,4 +15,6 @@ void ApplyArgsManOptions(const ArgsManager& argsman, MemPoolOptions& mempool_opt
     mempool_opts.check_ratio = argsman.GetIntArg("-checkmempool", mempool_opts.check_ratio);
 
     if (auto mb = argsman.GetIntArg("-maxmempool")) mempool_opts.max_size_bytes = *mb * 1'000'000;
+
+    if (auto hours = argsman.GetIntArg("-mempoolexpiry")) mempool_opts.expiry = std::chrono::hours{*hours};
 }