Fix run-pass/coerce-reborrow-imm-vec-arg.rs

[emberian]

I get

 rustc coerce-reborrow-imm-vec-arg.rs
coerce-reborrow-imm-vec-arg.rs:18:8: 18:9 error: cannot borrow const vec content as immutable
coerce-reborrow-imm-vec-arg.rs:18     sum(y)
                                          ^
coerce-reborrow-imm-vec-arg.rs:18:8: 18:9 error: unsafe borrow of aliasable, const value
coerce-reborrow-imm-vec-arg.rs:18     sum(y)
                                          ^
error: aborting due to 2 previous errors

Which I think is correct behavior. @nikomatsakis?

The test otherwise runs fine.

[nikomatsakis]

that is the correct behavior. I think the commented out section can just be removed from the test. it used to pass because of the purity rules, but those are no longer.