Remove obsolete error message regarding do-blocks with internal iters

When using a `do` block to call an internal iterator, if you forgot to
return a value from the body, it would tell you

    error: Do-block body must return bool, but returns () here. Perhaps
    you meant to write a `for`-loop?

This advice no longer applies as `for` loops are now for external
iterators. Delete this message outright and let it use the default error
message

    error: mismatched types: expected `bool` but found `()`
auto merge of #8561 : kballard/rust/do-block-internal-err-msg, r=thestinger

When using a `do` block to call an internal iterator, if you forgot to
return a value from the body, it would tell you

    error: Do-block body must return bool, but returns () here. Perhaps
    you meant to write a `for`-loop?

This advice no longer applies as `for` loops are now for external
iterators. Delete this message outright and let it use the default error
message

    error: mismatched types: expected `bool` but found `()`

r? @thestinger