Revert "ci: disable link-dead-code in coverage build (#8118)"

[andresilva]

This reverts commit 4d1cb01.

#8118 (comment)

This will break the coverage build but at least it won't report an erroneous coverage percentage.

[General-Beck]

let's leave this option

[5chdn]

What are we doing with this now? Hoping it will be fixed in Rust?

[andresilva]

There are two crates that break the compiler with link-dead-code (only one shows up in the build logs):

• futures-timer - I plan on refactoring parity-reactor to use the new tokio Runtime (https://tokio.rs/blog/2018-03-tokio-runtime/). This will give us more efficient global timers and we can then drop the futures-timer dependency.
• patricia-trie - I tested and this is fixed with Rust 1.26 (which should be released in a couple of weeks).