Remove references to std::pipes from task tutorial

This module was removed a while ago, but the tasks tutorial wasn't
updated, and the old docs page for pipes was never deleted so the link
confusingly still worked!
auto merge of #9223 : sfackler/rust/tasks-fix, r=catamorphism

This module was removed a while ago, but the tasks tutorial wasn't
updated, and the old docs page for pipes was never deleted so the link
confusingly still worked!