。
You don't need to bring a bento.
Literal
Bento [obj-を] take-go need [topic-は] not-exist (polite)-よ.
The 'don't need to' construction: ~必要はない ('there is no necessity to') — 必要 ('necessity') as a noun with はない as the negative existential. The topic は softens the negation slightly compared to が. 持っていく ('take [with you, away from here]') is the outbound counterpart of 持ってくる.