。
Leave out the useless ones.
Literal
Useful not-be things [obj-を] omit-[command-なさい].
役に立つ ('be useful,' lit. 'stand in service') is a fixed collocation — 立つ ('stand') here takes its usefulness sense. The negative 役に立たない ('useless') modifies the pronoun-like もの. 省く ('omit, leave out') is transitive.