it 的成分问题

False theories can yield true results, so even if an experiment works, it doesn’t prove that the theory it was designed to test it is true.

请问老师 句子中的这个it充当什么成分？

假如去掉这个it， 意思和原来的句子是否一样？

it doesn’t prove that the theory it was designed to test it is true.

红字的it是多余的，致使句子结构错误，需要删去，得到：

it doesn’t prove that the theory it was designed to test is true. 划线部分为定语从句。